They were only making remakes of g1 and g2 to bypass the gameboy/GBA split preventing transfer of data from g2 to g3, ensuring that all non-event g1 and g2 mons could be caught easily. It is fully possible to get all mons from R/S/E to BW and BW2 via a series of one-way transfers using a g4 game as intermediary, eliminating the need for a remake.
Besides, simply by in-universe timeframe, the games run FRLG/RSE -> HGSS/DPPt -> BW -> BW2. Reintroducing RSE would throw those first couple pairings off.
Not that I wouldn't want an Emerald remake. And only Emerald would do, for me. Gens 1 and 2 could get away with split remakes, since their events are analogous, but the triple-features in Emerald and Platinum would: 1) be ignored in favor of the original splits, bumming me, 2) be integrated into a paired release, being redundant, or 3) be released as a singular remake, which'd throw us for a loop.