Almost all healer NPCs are conjurers simply because that's what there are most of in Eorzea. White mages are very rare and almost exclusively padjal. Scholars are nigh extinct, with the WoL and the occasional tonberry being the only modern practitioners. Sharlayan astrology never really caught on in the rest of Eorzea so there just aren't that many astrologians outside of Sharlayan. By default that leaves conjurer as the primary healing class in modern Eorzea.