Because something with a cryptographic hash, like sha256, you can do millions (or in some cases billions) of hashes per second on the same GPU.

Password hashes, like bcrypt, PBKDF2 and scrypt, are massively slower. That doesn't mean they're uncrackable, it just means they are expensive to crack, so a strong password in a well implemented password hash will take a long time (and cost a lot of money) to crack, by which time the user has hopefully noticed and had time to change their password.