Because there's an actual LPIV now. Or will be very soon, anyway. At the time, Omega was intended to be the closing game of the Lemmings Plus series, but now that the series has been resumed and an actual LPIV is being made, it didn't make a lot of sense to me to keep classing it as a main game. (And if you want to be technical, Holiday Lemmings Plus was the one to introduce 7 of the 8 new skills; it didn't have any levels that used Mechanics but all the other 7 appeared in it.)
I can assure you however that Omega will be getting talismans too, as will HLP and the two bonus packs. It's only Flashbacks that possibly won't, though I may also consider copying over to it any talismans relating to the levels included in it. As far as Lemmings Plus I goes, there are only two levels that appear in Flashbacks that have talismans associated with them (Wimpy 10 "Labyrinth of Despair", complete with no more than 3 of each skill, Gold (and also the hardest talisman in LPI); and Medi 16 "The Oddstack", complete in under 3 minutes, Bronze).
The Wimpy 10 talisman was actually inspired by the Lix version of this level (I believe it was remade by Proxima?), which only has 3 of each skill - but it adds a few more skill types, also with 3 each. The added skills are Jumpers, Batters, and most crucially, Platformers. In actual LPI, of course, your only constructive skills are builders. Initially I had that talisman as four of each skill, but then I got an idea of how 3 of each skill might be possible even without the extra skill types Lix gives you, and spent quite a while trying to find a way - and eventually found one. The solution I found uses the full 3 of each skill, although since it saves 92% (two lemmings more than required), you can reduce the number of floaters used - but you'll still need the full 3 of everything else. That talisman is right up there in difficulty with the Fierce levels in LPIII, if not harder.