The reason there is no ZIII RAM card is because there is very little demand and certainly no investment.
If 200 people paid $200 up front, with a lead time of six months, I'd make one. Hell, any of the Amiga hardware developers would. It aint going to happen though.
Look at the costs: a design would cost about $4k make the first, and about $20-30 each to make there after with an MOQ of 200.
At $200 each, the break even is 60. If you sold all 200, it's $20k profit, not bad, probably worth the risk.
If you sell at $100, break even is 142 units and even if you sell them all you only make a profit of $4k. Hardly even worth the effort? There is also a very high chance of loosing the majority of your investment ($10k) :-(
The fact is that if you need more RAM you get an existing accelerator on the second hand market, Cyberstorm, Apollo etc.
If you are asking "How come there are no new Accelerators with RAM?" now that is a different question.
Someone said "136Mbytes on a classic Amiga is not enough". Really??
I think 99.9% of Zorro III capable Amiga owners would think a RAM board was cool, but when it came to putting their hand in their pockets, not buy one. Probably save up for a CSPPC, or a DENEB ;-)