I was surprised too at first (although I already knew GBJam 9’s ‘winner’ was a Clickteam Fusion 2.5 game), but then, thinking about it, it seemed logical: as I discovered in detail by trying myself, actual Game Boy programming is more demanding, as you have many more constraints than merely having four colours: you have all the code optimisation to care about, the sprite limitation (I think 40 on screen, and 10 by line if you want to avoid getting unvoluntary white), the tile-based VRAM graphics, the four-channel audio…
There is a game jam dedicated to genuine Game Boy games that is called Game Boy Showdown. :) It ran in May 2022; I searched information about other editions, but am unaware of them. I like the idea of really asking for true Game Boy games, so I understand your disappointment, but there may be various such game jams! I intend looking for them.
(For the record, I tried partaking with a true Game Boy ROM as an opportunity to learn Game Boy development, but unfortunately, I had big time problems, so my attempt is currently a one-day half-baked prototype of a minigame. Remove the word ‘half ’ and replace it with ‘quarter’. X) I have still uploaded it though not submitted — I am wondering if I should ask for the late entry form, just in order to have some very limited technical feedback and to be able to cast votes —, if you want to take a look, but this is really not much, although I noticed some people still decided to submit their work when it was unfinished; I did not dare to!)