Job Availability is always based on lore and not overall job representation. There are technically four healer jobs/classes available: Conjurers, White Mages, Astrologians and Scholars. Out of those four, Conjurers are the vast majority simply because Conjurery has the least amount of requirements needed to learn it. For starters, a Soul Stone isn't needed and there aren't any unsurmountable cultural limitations on who should learn it. So most, if not all healers seen outside of specific job quests are Conjurers by default. That's just what most, if not all, Eorzeans think of when they think of a healing mage.

White Mages are the most restricted. In order to be a White Mage, you either need to be born a Padjali or have the one known White Mage Soul Stone in existence. On top of that, the Elementals have expressly forbidden the teaching of white magic to anyone who doesn't match those requirements.

Scholar is similarly limited, but for different reasons. Scholar's faeries live in the Scholar Soul Stone and so far we only know of two of those types of Soul Stones. One is yours. The other belongs to Surito Carito from the Lvl. 60 - 70 SCH quests. Provided more Scholar Soul Stones are found, there could eventually be more Scholars. In fact, that's the overarching plot of the Scholar quests. Alka Zolka wants to revive the job and is actively studying it.

Astrologin should have more practitioners then White Magery or Scholar does. However, all those practitioners would be in Sharlyan and Sharlyan is infamously xenophobic when it comes to spreading it's own magic arts around. In fact, the Lvl. 30 - 60 AST quests are all about how the Sharalyans who want to introduce Astromancy to Eorzea are being hunted by a group of Sharlayans who don't want their native magic arts getting out. So if there's anyone who should be see using Astromancy it would be someone with a tie to Sharlayan. Although, given how the Lvl. 60 AST quests ended, it might be possible that the Ishgardians start using it sooner rather then later. All that said, we shouldn't be running into very many people who know what Astromancy is, let alone are competent at it.

Healers are not the only role that works like this. We see many more Muraders and Gladiators then we do Warriors and Paladins. And we see more Warriors and Paladins then we see Dark Knights. Likewise, nearly all damage oriented mages are Thumaturges. We see more Ninjas then Rouges and more Lancers then Dragoons. Puglists are more common then Monks, Samaurai are only seen in the Far East, we never see Machinists anywhere but Ishgard, and we've yet to see a Bard outside of the Bard quests even though we've seen plenty of Archers.

For better or for worse, the FFXIV team has never cared about "equal job diversity". What they care much more about is "accurate job representation". Once they determine how rare or plentiful a job is according to the lore, they tend to stick with it. Which gives a better world-building experience overall.