Resources. Intel has more MUCH more resources and has historically not allowed AMD any opening, except for the P4 debacle. That was when AMD did best and Intel played the most dirty. Intel's execution has been noted even outside semi industry circles. Think NV30/R600.
They have a process advantage that nobody in the entire semiconductor industry can match. (Or has matched in last ~15 years). All GPU's are fabbed in TSMC.
They have a cache density which is better than anyone else's (except edram, ofc. But that's a pie in the sky for gpu's). GPU's use standard mem cells.
They have enough employees to do a new uarch every 2 years while AMD can do 1 every 4. GPU's have evolved from ff hw, so there was not much of an uarch to innovate upon as such in the beginning.
CPU bug fixes are expensive as hell. FDIV/Barcelona TLB. GPU bugfixes (a la R600's AA) are patched in driver.
AMD will be lucky to gain *any* marketshare from Intel of today. GPU marketshare has never (to my knowledge) been as skewed as 80:20, let alone 90:10.
AMD has never sold >$250 cpu's (if you exclude the athlon hey-days). AMD served cake when NV surrendered >$250 gpu market.