There were several reasons given for the class changes. The devs felt that skills were losing potency the more they were added, there were many skills not being used, class builds were too much alike, the combat tempo needed more pep, animation times were sometimes too long, and it was just taking a lot of time for the devs to try to keep everything in balance.
"These are big changes," the devs state. "They take some getting used to. We know this. We also know that for the good of the game and its longevity, we sometimes need to make big changes."