The 600 is hard drive "ready", as well as the PCMCIA slot, so it is more readily expandable.
Expandable with the three possible expansions available for it, SRAM, card adapter, and Ethernet. That limits it, and you'll have to choose one and only one. Forget fastram if you need a transfer card, and forget internet if you don't have an accelerator (same as for A500).
Yes, for any accel to sell for A500s it would need a harddisk interface. 68020 accel without harddisk interface... no point in bringing out the A500 from the closet, there will be endless support tickets if misc old harddisk interfaces are used.
Also comes with ECS and 2.x kickstart, so might play nicer with some hardware too.
I doubt A600 sales came close to A500+ sales, and it has ECS and kick 2.0. So.

Not that ECS/2.0 ever played nice with anything...

The first thing people would do, now that new harddisk interfaces with fastram (and now maybe also faster CPU) are being made, is to finally plop in kick 3.1, because the harddisk interface would have drivers not made for kick 1.3

Im sure if someone made it for the 500, people would buy it, just because it was there, but I can see why the 600s and 1200s were first in line.
If someone made it for A500 (with harddisk interface), people would buy it just as they did for A600, because it would make it exactly as usable as their A600.
Sorry for butting in and respect to all involved, but I just don't buy how the A600 has any points in favor of the A500 in this particular question.