Diamond/Pearl/Platinum all released on DS, an extraordinarily popular system that almost everyone owned. They were some of the best selling and widely played Pokemon games ever, and it is still very easy to get a copy of the games. Especially in Japan, where older games take longer to stop being supplied. You can walk into a Gamestop and you'll still probably find the game there. In Japan, I can only imagine it's the same deal. They also are very close to modern Pokemon, as they released after Wifi. The 3DS is also still being supported by Nintendo, and is compatible with D/P/Pt. The average person interested in Pokemon can play the game with next to no issue.
Comparatively, Ruby and Sapphire released on Game Boy Advance, which, while popular, sold considerably less than the DS. By 2013, when ORAS likely began development, it was no longer reasonable to assume local retailers would have copies of the game readily available. The DS was no longer really supported by Nintendo, and the 3DS had no compatibility with Game Boy Advance. Certain Pokemon in Gen 3 were difficult to obtain legitimately at that point, as those games were not released with Wifi, and some didn't return often for later titles.
If people want to play Gen 4, they can find the original extremely easily. Also, FireRed and LeafGreen are older than D/P/Pt, and are pretty much guaranteed to sell way better. They also are not compatible with Wifi either. If Game Freak wants to continue being traditionalist, they will remake Gen 1 again. If they want to break the mold, they will likely focus their efforts on making an evolved Pokemon game with higher production values. Either way, Gen 4 is not a priority.
Plus, ORAS was likely a much easier game to develop, since you could very easily reuse assets from XY on a popular console where revenue was basically free in comparison to developing a high budget Switch title. The Switch is a perfect time to start breaking out of the yearly mold, and remaking Gen 4 instead of Gen 1 doesn't really make any sense.
Way more people are interested in a gigantic revamp of Gen 1, or a new take on Pokemon.
Addendum:
i also think sinnoh is a really poor region in general. not only did d/p have problems with game speed, hm bloat, and poor type balance, the region is a nightmare to explore and has lackluster aesthetics. it also didn't capture the feeling of adventure like the games before it. it's too mellow and the plotline is too out of pocket for a pokemon game.