A dearth of lore
When jewelcrafting was introduced in The Burning Crusade, it came with this new species called the Draenei, who had lots of connections to jewels and the magics they could contain. It made sense that everyone in Azeroth would start learning this new and mystical profession with their arrival on this world.
But inscription is less clear. To me it feels as though it's been around for a long time, perhaps as a long forgotten art form, and it's only recently been discovered again. Although we don't know many characters in the Warcraft lore who have identified themselves as magical scribes the way our characters can, we do see a number of characters who use "runes" and other forms of magical writing. Death Knights and the Lich King himself are the most obvious examples, but Iron Dwarves also use runes of various sorts to bring about magical effects. The inscription we can all learn as a profession may be related to that in some way.
On the other hand, we could say that there are significant differences between runes and glyphs as well. It may be that a death knight's runes are pure evil (like fel magic), while glyphs are more neutral (like arcane). Also, the death knight's rune interface in the game actually uses symbols that look more like modern-day icons rather than cryptic magical writing of some sort. Perhaps death knight runes are more about necromantic energy than they are about letters in some ancient and evil language.
An ancient language
Personally, I prefer to ignore the in-game icons in the death knight interface and assume that the actual death knight runes look more like the original ones used in the Wrath beta (and now in death knight addons such as "DKiRunes"). I also assume that they are letters in some ancient writing script whose origin is not clear. It could be the language of the Titans, the Old Gods, the demons, or even the ancient elves from before the sundering of the world. Even the Lich King himself, I would say, is only partly aware of the full meaning of the language he uses in his necromantic magic.
I would also argue that this language was lost for the most part on Azeroth, except in Northrend. The Lich King probably discovered it years ago when he was originally banished in the northern continent, and bent its magic towards his aim of raising a massive army of undead. Later on, Dwarven expeditions would have had a chance to head north and discover these runes in their archeological excavations. Having brought this knowledge back to the other races of Azeroth, the peoples of the world could have learned how to use these magical symbols in ways that are totally unrelated to the Lich King and the undead. They could discover that, just like English words can be used to promote enlightened understanding as well as bigotry, the words of the ancient language can be used to improve one's Fireball spell as well as channel undead energy through a death knight's runeblade.
So from my point of view, inscription glyphs and death knight runes are basically different words the same language. The difference between scribes and death knights lies in how they use these symbols more than in the symbols themselves. If you look at the original death knight symbols, they even look similar to the glyphs, which helps to support this idea.
The art of inscription
If you consider other ancient writing systems with pictorial elements, such as Egyptian hieroglyphics or Chinese characters, there's always some element of art involved in the actual writing which goes above and beyond the actual meaning of the word itself. Hieroglyphs were presented with great clarity and color, while Chinese is presented in more abstract forms, often flowing from one stroke to another like streams of water. It stands to reason that a decent scribe in Azeroth would have to do more than just write a glyph in any old way, but would have to inscribe it very carefully, with the utmost attention to art and form, in order for it to have its magic.
When I imagine what it must be like to be a magical scribe (as opposed to just a regular scribe who would take notes at meetings and such), images come to mind of our characters studying old tomes long buried and forgotten, covered in dust and ancient writings few living people now understand. As the difficult translating work goes on, people discover more and more tattoo-like markings which have a magical power of their own when written on or transferred to a person's body.
Being inscripted upon
I'm only assuming that glyphs are written on the body itself, of course, but actually it's an open question not yet clarified in the game or in the lore at all. Are they written on your weapons or armor? Do you carry around sheets of paper with these magics written on them?
If they are written on the skin, a master scribe could write them on a magical sheet of paper, and then adventurers like you and me could just apply the glyph to their skin so that the glyph could transfer it over. These glyphs would then stick to the skin like tattoos, which could be either very discreet, hidden away under the clothes, or else displayed openly wherever one likes.
One glyph might make a person slightly better at shooting a particular type of arrow, while another might make a shaman's lightning more potent. Many would seem to have no effect at all if they were worn by someone with no actual ability in the designated area. Perhaps a typical scribe might seek willing (or unwilling subjects) to practice writing particularly indecipherable glyphs upon, in the hope of discovering what they do. This could be something we do while we level up the profession.
All this is only one of multiple solutions to the problem of adapting inscription to the lore of Warcraft. If you prefer to say that the inscription your character studies is more of an artistic thing than anything involving ancient texts, then feel free to make it your own branch of the art. If you feel like the artistic element shouldn't matter, then there's room for that too.
One could imagine other possible origins for inscription as well -- are all glyphs cursed because of some connection to the Old Gods? Have they been around in Azeroth all along? Perhaps it doesn't really matter that inscription was only added to the game in Wrath of the Lich King, if we all just agree that it's been around all this time as far as the story is concerned. Virtually every aspect of this profession can be re-imagined in one way or another.
How do you fit inscription into the lore and into the life of your character?
All the World's a Stage concludes this series on roleplaying within the lore with this week's look at being a scribe. Be sure to check out previous articles on roleplaying engineering, leatherworking, jewelcrafting, blacksmithing, enchanting, skinning, herbalism, mining, tailoring, and alchemy. With every race, class, and profession covered at last, is this series actually finished?