The reason for this is likely twofold. Firstly, the boss was very hard to kill, and players were not unfairly deciding that the cost in repairs and time of killing Oondasta was too much compared to the gear drop chance and the quality of the gear. With patch 5.2 there are easier ways to get gear than killing this boss.
Secondly, the huge numbers "required" to kill this boss meant that servers were struggling under the load of hundreds of people taking on this boss. One viable "tactic" was to get so many people attacking Oondasta that the server lagged out so badly that he was only using a few of his abilities with long intervals in between.
Rygarius said a while back that while Oondasta was great, in that he brought back the epic outdoor raid feel, but that the big groups were causing server stability problems, so this is likely another step in trying to fix that issue. It seems like the nerf, meaning that the graveyard zerg tactic would likely not need 200 people to succeed, will make players happy to pull with smaller groups. This will help with the server stability, but is it a worthwhile exchange?
Hit the break for Lead Encounter Designer Ion Hazzikostas' input on the change.