I would guess because an FPGA-based accelerator offers essentially no advantage over one using an actual 680x0 chip and requires much more time and effort to design (look at how much effort has gone into NatAmi's CPU core.) If you're designing an FPGA-based system anyway, sure, it makes sense, but if you're just trying to get an accelerator for an existing Amiga system, it's a lot simpler, cheaper, and less time-consuming to just use an actual chip.
True, but FPGA is not as limited in terms of potential performance. It wouldn't necessarily be "just an accelerator," it could be the "mother of all 68K accelerators." Could sell better than PPC accelerators IMO. Imagine an incredible power-boost for your classic Amiga that is still largely compatible with old software.
