The original games are so old that it is quite messy to get them to run on PC these days and the remake changed the gameplay quite substantially. So this remaster is quite welcome and probably a lot cheaper to produce than a full 2 and 3 remake.

As for Anniversary, the story there is a bit weird, as Anniversary and Legend are basically completely independent games that have nothing much to do with each other other than the engine. Legend is a reboot and Anniversary is a remake. It's only with Underworld that the the story of those two get wrangled together into a trilogy. Only the first game is taken into account there, all the other sequels of the original game are ignored.