Purely for comfort reasons. The HD650 (and the 600's for that matter) IMO are superior to MA900 in nearly every way sonically, but with my TMJ, their clamp and the shape of their cups (oval cups of this size always gives me pain, circular pads like MA900 and K7** agree better with my jaw) I could never wear it pain free for more than an hour. I really love the HD650's sound and was actually stubborn enough to buy it on THREE separate occasions to try again and again to somehow get it to work for me, but I could never solve that comfort problem... and yes, I've tried bending the headband etc, but I eventually realized it's mainly just due to how the cups line up to my jaw, as even a weaker clamp would start to hurt my jaw after a while...and even more telling was the fact that I found the LCD-2's, which actually clamp MUCH harder than the HD650s, much more comfortable on my face and jaw due to their bigger rounder cups and wider clamping pressure distribution. It really sucks when you finally find a headphone with a sound signature you feel is pretty much perfect for your own needs, but then find that you can't wear it for extended period of time without pain
...
One day I may become daring enough to try a more drastic mod on the HD650 to try to improve the comfort enough for my needs somehow... but for now, the MA900's have been an acceptable substitute with it tonally being close enough to HD6**'s (somewhere in between HD600 and HD650 actually, IMO) and with its second-to-none wearing comfort.