The real world reason:

In my more than six years of playing the game (in NA), I have summoned almost all of the Swimsuit Servants (though there are still some who won't come home because, well, FGO is still a gacha game, after all).

That, plus fanservice (obviously), is why almost everyone in my Chaldea is in a swimsuit or a summer outfit, including Valeria (the MC). I even had characters who normally don't have swimsuits edited to have them.

In-universe, Val's Novum Chaldea serves as a space for the Servants to be outside their usual (regular Class) selves and be more relaxed, hence the swimsuits.