The DE10-Nano itself is a bit large for a handheld device, and hasn't been optimized for power consumption. (It's designed as a development board, not as a component of a finished product.) There's nothing stopping someone from using the Cyclone-V SoC in a handheld device, though.

I’d reckon it’s the same reason that there isn’t much of a custom laptop scene. The open ended nature of stuffing a screen, battery, and input peripherals into a chassis seems an order of magnitude more difficult than just making a headless box to plug into your TV.

But with some effort, it would be awesome.

I think it is because the motivations for retro console FPGA simulations are to play competitive action games sit down with highly accurate timings. “There is a 5 frames window after this triple input sequence” thing. Pocket gaming happens at much relaxed timings so less demands exist for low latency cycle accurate simulations.

The limited market is problably covered with Odroid Go / GPD XD / RG350M etc. Mister leverages an off the shelf FPGA board that would require a lot more work in a handheld form.

It was being worked on at one point. I forget who was doing it. They showed off mockups on smoke monsters streams, but I havent seen much of it in over a year.