Some of it might be ease of printing. For the 1990s Omni, a lot of that material seems to have been reproduced a lot lately, much double dipped in other omnis. So there is no need to remaster or sort out rights. For others. I think the Marvel Now Omni was trying to pull off what DC does but it didn't work out, and hopefully we won't see that for All New All Different. Ultimate marvel Omni was all material that had already been set in Omni and OHC so it was probably fairly easy to slap together.
Marvel still makes an effort to get licensing and old stuff ready. Just not at the rate I personally would like.