Even among people who are strictly asexual there is a spectrum. Some asexuals are completely sex repulsed and do not even wish to hear about sex. Others may be interested in having sex, despite not feeling any sexual attraction.
There is not just one way to be asexual, so why is it so hard to fathom the existence of grey-asexuality and demisexuality. The defining characteristics of these sexualities is that they are, for the majority of the time, functionally asexual. THAT is why they are included in the asexual spectrum; not because they are trying to take over, but because for the most part they identify better with asexuals than with any other group.