Because I can.
It's new. It's volatile. Its value fluctuates. It's a pain in the ass to convert. It requires I spend hours/days/large quantities of time I could otherwise have spent working for my actual currency to figure out how to price it, use it, etc. It's a gigantic pain in my ass.
99.9% of paying customers are ready and willing to pay me in such a manner that I get my local currency--directly, with no added work on my part, and no fluctuations in value.
So, the real question is, why should models accept bitcoin? Because I have a motivation of approximately zero, and I feel fairly confident most models will agree with me.
If someone wants to convince me to accept bitcoin, they'd better get to work selling me on it and no one has to date.