People have X amount of cash so if they shoot 50 cal or 68 it makes no difference, if they have £50 to spend on paint that's what they will spend.
I think that's the main problem for people who have already invested in .68 (as opposed to new players).
.50 isn't cheap enough to make sense for me. I play once a month tops a usually shoot a box in a day. I've got five .68 markers which I've collected over the years which cover the whole range of play from stock class pump play to tournament.
To swap to .50 I'd need a new marker, new loader and new barrel kit - say £400 for those three as a conservative estimate. According to the figures quoted on here, tournament grade .50 is £7/box cheaper than equivalent .68 paint (both retail prices).
To recoup my initial £400 investment would take almost 5 years before I started saving money and I'd be using marginally inferior paint for that whole time.
It might make more sense for new players, gun whores and people who shoot a lot more paint, but not for me.