Because Bitcoin is mined on ASICs. It's not a graphics card, instead a single chip optimized for one function only: mining (by generating hashes).
GPU based mining is mostly done to mine Ether. Ether will switch to Proof of Stake somewhere in the next 1-2 years, after which the competition between gamers and miners for GPUs will end.
That's the optimistic outcome, the pessimistic outcome is the miners will fork "eth-classic" and keep going. It's up to the reader to decide which is most likely, or both.
Or they will move to some other coin that is rising in popularity and doesn't have GPU resistance built in. The latter is something that feels like a negative these days if you want a lot of buzz about some new crypto tokens...
You could mine bitcoin on them, but it would be so inefficient compared to ASICs which can hash many magnitudes faster, you would have an astronomically small chance of mining a block.
