Festerblight is a play style that's been around for a while, but has come into vogue in patch 5.2 especially. To put it simply, the idea is to stack as many strength and attack power buffs as you can, and once your strength is at its peak, to apply your diseases, then use Festering Strike to make that set of diseases last as long as possible. Since diseases take a "snapshot" of your stats at the time of application and base their damage on this number, you can stack your temporary buffs as high as possible and make them last through an entire fight. For example, an Orc death knight might wait for a Fallen Crusader proc and a Fabled Feather of Ji-Kun proc, then stack Potion of Mogu Power, Synapse Springs, Blood Fury, and a Tricks of the Trade buff from a rogue guild mate. This gives him an incredibly high amount of power. Then he applies his diseases via Outbreak, and keeps them up by using Festering Strike constantly, even with death runes that would otherwise be used for Scourge Strike, switching back to Scourge Strike only if the diseases accidentally fall off or will not expire before the boss dies. Festerblight is, in many ways, a thing of beauty. It's a way to take burst damage and extend it out to infinity, capturing a moment in time. It's almost poetry.
Now, there are disadvantages to this play style. For one, you won't do well with this play style on bosses that require target switching or otherwise have places where dropped diseases are unavoidable, such as Iron Qon. For another, it goes to waste if you accidentally overwrite your diseases with a lower damage version or let them drop off. Still, since festerblight still generally uses haste and critical strike as its major stats, you can easily go back to a "normal" unholy rotation until it's time to line up your procs and try again without losing too much damage.
Thus, we see the real reasons for the nerfs. Unholy Might, of course, buffs diseases straight up, so dialing it back helps that. Dialing back Festering Strike damage means it should be no longer worth it to sacrifice those Scourge Strikes you could used. It is certainly possible this will spell the death knell for festerblight.
So why destroy festerblight? In many ways, it's simply going off to the same dustbin that diseaseless blood went to. Blizzard needs to be able to balance classes around expected modes, and allowing "alternate" styles of play like this just makes balancing that much more like herding cats.
Of course, you could argue that masterfrost, a similar mutation of an established DPS tree, was essentially allowed to stay in game, legitimized by becoming the dual wield damage rotation of choice. Festerblight comes from the very real and solid unholy tactic of making sure diseases are as strong as possible, so shouldn't it get a similar lease on life? The difference here, I think, is that masterfrost serves a very important function that Festerblight does not. Balancing two-handed frost and dual wield frost has been a perennial issue for Blizzard. By giving two-handed and dual wield frost two separate rotations, it leaves them free to have more levers to balance the two, instead of simply buffing and nerfing Might of the Frozen Wastes and Threat of Thassarian and hoping it all balances out, as they have in the past. Paradoxically, then, masterfrost makes it easier to balance by establishing a separate mode of play for a supported weapons loadout, while still supporting all the major buttons frost should use. Since Blizzard is dedicated (rightly, I think) to keeping both dual wielding and two-handed weapons as viable choices for frost, legitimizing masterfrost helps that.
Festerblight, on the other hand, adds an extra layer of complication to a tree that only really needs one rotation, and attempts to nearly completely shove out the signature move of Scourge Strike from the rotation. This makes it much closer to diseaseless blood (which tried to remove the key rotational element of diseases) than masterfrost (which still requires Obliterate as a cornerstone of the rotation, if only to convert runes). With this in mind, Blizzard's decision to kill the festerblight variant does make sense given previous design decisions.
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