Mutliclassing in 5e is a half baked optional rule, the land of broken (mostly very underpowered) combos. One good reason for Larian to avoid it is it gives players without 5e system mastery a whole lot of trap options to fall into.
Multiclassing is certainly broken and implemented with serious issues. It's broken in both ways, making for severely underpowered and highly oppressive combinations. Ranger/Rogue and Ranger/Druid make regular singleclass Ranger kinda pointless, for example, while Wizard/Cleric combine very poorly by cutting off access to high level spells.
But it is also a very popular feature that Larian have stated they were going to add.
