Can someone give me an in-depth explanation why Summoner is arguably the worst DPS?

[Aikaal]

Then explain it to me. Blm goes though fire phase to dish damage, then ice phase to replenish mana to repeat the process. Having a larger mana pool on blm Mage means squeezing out an extra fire spell during astral fire phase. And while it affects your incombat mana regen (pre umbral ice stacks) the amount you regen with umbral ice stacks makes that amount negilable.

Fire I is 306 Potency. It also has 40% chance to give you an instant cast free Fire III that is 468 potency. So if you had more Fire I in your rotation, you do a lot more damage. In your ice stance, Blizzard I is 170 Potency and Thunder I is 240 Potency. If BLMs were to have the same mana pool than SMN which is 3k, I'd barely have mana for cast 4 Fire I in my rotation. I'd be useless.

You could say I prefer to spend as much time in my Astral Fire stance and the least time in my Umbral Ice stance but I,d want to have full mana in my Astral Fire.