My understanding (fuzzy), is that 3e was trying to compensate for mages feeling some kind of underpowered in AD&D 2e, and the playtest went poorly / was in a limited, megadungeon combat grind low-level environment that was - and as a result they significantly underestimated the power of 3e casters at release, particularly the Druid and Cleric, due to the Druid and Cleric's playtesters playing them very suboptimally, a confluence of factors which led to an underestimation of the power and versatility of casters.