But why is the DM making a big deal about something they demonstrably don’t care about?Neither has to adapt and no one is forcing people with incompatible preferences to play together.
Presumably, the player and the DM want to play with each other. In your premise, the player wants to play a triton, but the DM’s setting only has sea elves. The DM’s only stated reason for not wanting tritons is that he finds sea elves and tritons fill the same niche.
So why not replace the sea elves with tritons? Why is any change to the setting considered “irreconciliable differences”?
Fun fact: the term “irreconcilable differences”comes from matrimonial law, where it was one of the grounds for divorce before “no fault divorce” was implemented. “Irreconcilable differences” was limited to reasons that were well, irreconcilable, not “I want to go to Hawaii on vacation” and “he wants to go to Colorado”.