Why no FPGA accelerator cards?

[psxphill]

Still, at that point you're harnessing a 150MHz CPU to a bus and chipset that still run at 3.5MHz;

You'd put ram on the CPU bus, so you wouldn't need to touch the motherboard until you access the customer chip registers. With write queuing then it wouldn't impact performance much. Reads are more of a problem unless you can execute instructions out of order.
 
The bus on AGA amigas runs faster than that as well.
 
Even modern PC's have multiple buses running at different speeds.

[psxphill]

any Zorro cards are running at a mere 2x the chipset speed. It's still better than an unaccelerated Amiga, it's just not as good a solution as a uniformly faster system.

It's only graphics cards that need a lot of bandwidth, everything else would be enough. I wouldn't bother with zorro though, big box amigas are too expensive. I'd be happy with an ubber fast A1200 with AGA, especially if a texture mapper could be built in.

[psxphill]

when you can fit the rest of the Natami (or FPGA Arcade/ Replay) cores on the same FPGA for little extra cost and end up with a far faster and more capable system that's still backwards compatible.

An fpga on an accelerator card with some ram is going to cost considerably less than a natami. Both in terms of materials and assembly cost.