Because it is still a sexual minority. Asexual individuals often are told that their sexuality isn't even real, that they just haven't met the right person, that they just need a good lay, that it's a medical issue they should get checked out, that they are just incels that want to feel like a special snowflake. In extreme cases, corrective rape is a very real thing that happens as an attempt to "fix" someone's asexuality. Asexuals are called selfish, unnatural, or freaks, for not feeling sexual attraction, because "it's part of being human" and "every person experiences sexual attraction unless there is something wrong with them". The type of oppression they experience is different from same-sex attracted sexual identifies, but that does not mean that they do not experience it.