That was a part of the reason as well, but when you get right down to it, the reason was quite simply that players were intentionally 6-slotting all their powers with a single enhancement type which varied depending on the power, which while great in theory could inadvertently gimp your build, and in some cases could greatly imbalance your build in ways the Developers never intended.
I used 6-slotting with damage for my example simply because without accuracy enhancements you'll almost never hit anything, and that's one of the things that the developers wanted to steer people away from.
I agree, but it wasn't just that it imbalanced the existing build, it was that future content couldn't be balanced without restoring a measure of predictability to the way existing powers performed. If your current ranged archetypes do somewhere between (random numbers here, I don't know how existing archetypes performed) 50-100 dps, depending on powersets, then you would know that the next powerset needed to fit in that range. But if perma-Hasten, 6-slotted Stamina, 6-slotted Targeting Drone and 6-slotted damage SOs meant that /Devices Blasters did 2000 dps, you're never going to be able to create anything else. Because do you balance future content around the 2000-dps freaks and leave everyone else frustrated, or do you balance around the 100-dps "expected" characters and leave the /Devices blasters bored to tears?
This, AIUI, is the problem Champions Online has with its freeform characters. Either you balance around the optimized freeform build, in which case the majority of powers become so sub-optimal as to be useless, or you balance around the hypothetical "normal" character, in which case the optimized build waltzes through content designed for whole teams. (Mind you, you can go too far the other way as well, which was the charge leveled against ED at the time--if nothing ever provides any advantage, then you're not really giving options, just the illusion of same. There's always going to be a better choice.)