Apparently HTC is not big enough to make 500K units a month for one model. They make over a dozen android phones alone if I recall. That probably impacts the output for one model than any other factor, except maybe dependency on parts supplies not in their control.
BTW, Epic is the 4G version of the Galaxy. Sprint needs to get that out the door now.
HTC is the most popular manufacture right now for smart phones. They had a similar shortage problem with the HD2, but soon fixed it. They haven't made a single phone that hasn't sold well. So maybe they need to figure out a way to keep up with the demand.
And Sprint is parterning up with Samsung for the next 4G phone. It's going to be for all major carriers. Sprint version will be called Epic, while the official name is Samsung galaxy.
I agree with the poster above. Samsung is unable to provide HTC with the AMOLED screens. Maybe it's part of their plan for the galaxy.
Edit: LG also makes them but it is unable to meet the demand because of shortage on their end.