[Eelongate]

> But this is motivated by stereometric reasons – everything on the same latitude is equally distorted

Expanding on this; Mercator is useful for navigation because it's conformal, i.e. it preserves angles. When you're plotting the course of a ship, this is very convenient.

[BurningFrog]

And historically, ship navigation was the dominant reason to make maps in the first place.

Only when maps were used for other purposes was there any reason to develop other projections.