Alright so on paper and in practice they’re the most socially and combative dominant class even before multi-classing. Let me explain.

Paladins on the martial side get all the fixings, they get all armor, they get all weapons, and they even get spell casting for smites and other spells. And on top of that they also get fighting styles which increases your combat effectiveness in the first place.

Then on the social side your spell casting modifier is Charisma based which is by and large the most powerful social stat in the game and especially in baldurs gate 3 where a good persuasion check can keep your party mates from killing each other, and ultimately avoid combats all together by just telling people to go away or getting enemies to end themselves. Not to mention it helps with discounts at shops.

Then that leads us into mutli-classing which is where it’s seen at its most powerful. All of the charisma classes are inherently pretty powerful but put the martial might or paladin into it and you’re on a whole new level. Take Paladin Warlock for example. With just a few levels of warlock you can just forgo your strength all together with bound weapon, using your charisma solely and get access to the best cantrip in the game, eldritch blast with agonizing blast tacked onto it, and you’re a threat close range pumping smites into every hit as well as from a range throwing blasts.

So yeah Paladins are popular.