TOS has endured for almost fifty years, and the reboot movies have renewed interested in that part of the franchise.
I wish there were more DS9 merchandise options, but I get why there isn't.
My guess (based on 5 episodes of Toy Hunter LOL) is either:
1. There are more TOS collectors out there.
2. CBS has it wrong and thinks there are more TOS collectors than DS9.
3. The TOS collectors are of an age that they want and can afford toys.