Because the running mentality was/is that it needs to be all cart/core system to be valid. If I had used the CD platform, it wouldn't have been considered "impressive". The same reason a SegaCD version wouldn't have been that impressive (general consensus).
That's just BS!
For a start, the PCE DUO is a "core" system (at least in my book), and the original PCE system was always designed to have a CD-ROM (as has been discussed-to-death in other threads here).
The Sega CD was an afterthought, and anyway, it's so crippled by the VRAM transfer bandwidth from the 2nd CPU that it's not like you get great video out of it as a CD streaming-platform anyway, and there's no super-fast connection to the ROM (IIRC) for it to read everything from there.
There's this mentality that "oh, a CD system is just streaming video/audio.. no big deal". Well, technically, the cart is just doing the same. But the cart has storage constraints, so I guess that makes it more impressive.
Exactly ... and a cartridge has an exponentially higher bandwidth than a CD, so you're just trading one limit for another.
As a programmer, you already know these things.
You also know (I assume), the huge differences between the primitive 4th-generation console CD-ROM hardware, and the 5th-generation console CD-ROM hardware (with video-streaming designed in from the start).
If you limit yourself to the pre-conceptions of idiots, then you just sink to their level instead of showing them that there is a bigger world out there.
That, and being a cart/hucard platform - you have a wider range of PCE fans who can run the demo.
I get the desire for an audience ... but then why not limit yourself to the standard 1MB ROM configuration and play to all of the PCE's weaknesses?