They are rereleasing the whole RE series on GC because the series is going to be licensed solely to Nintendo from now on. Since this happened, they didn't want to just drop RE Zero and RE4 on all their customers out of nowhere. So, to familiarize unknowing Nintendo fans with Resident Evil (and to make massive profit) they have decided to rerelease the whole existing series on the new console. That way, a person who has never played the games before now has a chance to play them in higher resolution and on a far faster system.