I believe they had intentions (could be wrong) to add in new ones at some point back in ARR but seeing as how Titan is worthless, Ifrit is more for pvp/bosses and Garuda is more used than the other 2 it just kinda fell apart.

As for the glamour that was also promised or hinted at a while back but got put on the back burner, but they managed to get out carbuncle glams instead.

Either way you're unlikely to get an official response from the devs here, if there's a post about it from them it would be more than likely buried in the smn thread on the Jp side.