The main reason for including Lemmings Redux rather than the (mostly) unmodified official packs is that a lot of the official levels were either bad levels in the first place (builderfests like Hunt The Nessy), or just don't work as well under NeoLemmix (for example, any level that revolves around bomber timing or other execution difficulty factors, don't hold up so well when bombers are instant and gameplay can be framestepped / rewinded). It's also an opportunity to showcase some of the less-known official levels that are really good, such as many from the Genesis version of the game. Add in the other minor improvements like fixes to particularly bad backroutes and the greatly improved ordering of levels, as well as that Redux applies many of the modern fanmade content standards (like no unnecessary time limits), and yeah, just didn't make sense to go with the direct conversions instead of Redux.
Another issue is the disputes around the "authentic" way to convert them. Some conversions (like the ones Armani linked above) mostly stay as a direct conversion but trim unused space at the level sides. Some don't even do that, and only make modifications where physics differences in NL make the level impossible / significantly harder than it should be. And some won't even do that, expecting just a direct as-is conversion. There's the disagreement as to whether Fun 6 should retain blockers, given that the point of the level is to teach the player about Bombers, and Blockers were only provided due to the bombers being timed in the original versions (which they aren't in NL). There's also levels with regular terrain overlapping steel in ways that are intended purely for decoration such as Tricky 4 / Taxing 7 - in NeoLemmix, this would result in a weird mish-mash of steel and non-steel pixels, so there's the question of if/how to deal with that.
Lemmings Redux works well, as it's a good bridge between the official games and NL fanmade content - you get mostly the levels you're familiar with (but a few you might not be thrown in, eg. those from the Genesis version, if you haven't played that), while seeing the different standards you can expect in made-for-NL content (in particular around abuse of time limits, and keeping the lemming count to a reasonable amount where practical).
