Why are most jobs related to Ul'dah?

[Vyrerus]

I rather like Gridania as the supposed canon starting city. The call to adventure for Hydaelyn is far stronger from it, and also makes more sense coming from the mouth of the Elder Seedseer than say Merylwyb or Pink Puff.

She describes the mothercrystal to you in actual detail. Is a little more on board with getting you on board with your grand destiny.

Not too mention the little detail of the required item for the audience with them. Gridania's is a ritual mask, and it makes sense that you have to wear it because you are doing one of their forest rituals. Having to wear the earrings / shoes for Ul'dah / Limsa makes very little sense for either ruler to require and seems more like an inclusion so that Gridanian starters aren't feeling more special than other players.

Plus, I say that the WoL being allowed in by the Elementals at the start of their quest makes more sense for why the WoL is accepted later on without any fanfare or questions. While also being the first subtle hint at, "Oh hey, the WoL is special." Cause no matter who they are or where they're from, they're accepted into the Twelveswood and Gridania proper.

I say all this as someone who started in and very much loves Limsa Lominsa more than the other city states.

Edit Addition: Speaking of bookends though. Having a major leader talk to you about the mothercrystal and having it make sense helps to keep your mind on what that is, since it comes up again during the celebration in Mor Dhona at the end of ARR.

[Travel]

Mind if I post my two cents?

Taking a look at the WoL's "canon" jobs (Archer/Bard for Legacy, Warrior for ARR, Dragoon for Heavensward, Monk for the Gyr Abania side of Stormblood, Samurai for the Othard side of Stormblood, Dark Knight for Shadowbringers and Paladin for Endwalker) it seems quite likely that the WoL is supposed to be from either Gridania or Limsa. Judging from the cinematics, the WoL seems to have become a Warrior by the time Bahamut got unleashed; hence, given that Limsa is the place for Warrior training for ARR, it's likely the "canon" starting point for 2.0.

However, for 1.0, it seems more likely that Gridania is the starting location for 1.0.

[Cleretic]

I think to actually try to answer the OP's question, it might be good to ask why each individual job is based in Ul'Dah, to see if we can find common threads. So, asking that...

Thaumaturge/Black Mage: Mhachi descendants by all appearances migrated to the Thanalan; Black Magic is there because that's where most of the knowledge is centered.

Gladiator/Paladin: The actual coliseum, of course, requires fighters. The Paladins are closely connected, and have a history in Ul'Dah.

Pugilist/Monk: Pugilism I don't think has a specific reason, but Monk finds its initial roots there because of the Ala Mhigan refugee population.

Samurai: Musosai is based out of Ul'Dah, basically doing a gladiatorial show at the time we meet him. However, it should be noted SAM's storyline is very nomadic, and doesn't spend much time in Ul'Dah afterwards.

Red Mage: X'rhun Tia is in the Thanalan investigating a cult targeting Ala Mhigans; while RDM's storyline is also largely nomadic, a good amount of that initial story takes place in the Thanalan because of that Ala Mhigan connection.

Blue Mage: Doesn't start there, but quickly moves there due to taking up patronage at the Masked Carnivale.

Reaper: It might be that the Thanalan has more members of the Garlean diaspora there, but it's not clear if that's true.

So there's basically two leading reasons why jobs tend to turn up in Ul'Dah:
A: Immigration, which has been mentioned before. For Monk and Red Mage it's because of the Ala Mhigan community, while for Reaper it's Garlean and for Samurai it's sort of just the general drift towards the place as a seat of commerce.

B: The gladiatorial arena. This is actually a SIGNIFICANT draw, being explicitly responsible for Gladiator/Paladin, Blue Mage and Samurai, as well as being a likely suspect for Pugilist as well. And it makes sense; of course a sanctioned combat arena's attracting people who want to show off combat skills.

[Fenral]

The Pugilist story introduces Somnus very early, which factors into the transition from ARR to Heavensward. (And the de-transition from "edgy-mode" FFXIV to "lol jk like we would really" FFXIV.) It's probably the most obviously MSQ-relevant starter class even just for that.

[Lyth]

The Lemures migrated to Ul'dah to practice bartering with demons. No better place for that.

[MikkoAkure]

The Pugilist story introduces Somnus very early, which factors into the transition from ARR to Heavensward. (And the de-transition from "edgy-mode" FFXIV to "lol jk like we would really" FFXIV.) It's probably the most obviously MSQ-relevant starter class even just for that.

The very first quest you receive in Limsa Lominsa introduces you to somnus and won't even let you leave the "new character instance" to join the rest of the game until you complete it.

[Hasrat]

I rather like Gridania as the supposed canon starting city.

Nah, Gridania's too racist/xenophobic to get the spotlight like that.

That is, unless you wanna start categorizing "starting city" with respect to races as well.

At least, as a Duskwight, I surely wouldn't feel immediately welcomed to the city... Just as any player wanting to claim Ala Mhigan heritage probably wouldn't.

[RyuDragnier]

I rather like Gridania as the supposed canon starting city. The call to adventure for Hydaelyn is far stronger from it, and also makes more sense coming from the mouth of the Elder Seedseer than say Merylwyb or Pink Puff.

If it makes you feel better, it's most certainly the canon starting city for 1.0, considering Papalymo and Yda's entire arc is back in 1.0 in Gridania.

[SannaR]

And yet Gridania has the most quests compared to the other two before they start the whole envoy thing.

[Iscah]

And yet Gridania has the most quests compared to the other two before they start the whole envoy thing.

Quest numbers aren't a good metric alone – I would bet that they're shorter than quests in the other cities. Overall the early-game quests are very similarly structured.