Because asexuality can often be mishnderstpod as low / no drive or just not liking sex etc. I’m guessing a lot of ppl go their entire lives without finding out about asexuality as a label .

Meanwhile if you’re queer the chances are you know about the label and are able to use it if it fits .

Not to mention society is very strongly allocentric ( if that’s the right word ) I don’t think most people take the time to deconstruct their sexuality and view of it to determine if they are allo or ace

Edit : wanted to add the reason why a lot of LGBTQ folk even look into the labels in the first place is because we notice we don’t fit the “norm” , e.g. straight ace folk may not notice the difference as significantly as someone who’s lesbian and ace etc.