Multiclassing as feature shouldn't be a big deal to implement. It has a simple set of rules as D&D doesn't have any crazy math or restrictions in the background or even complicated rules, so this shouldn't be a big deal to implent on code side. It will require a rework of the level up UI, but that's about it as long as they did some minimal ground work required to separate spells (in case player wants to replace them on level up).
There are a couple of reasons why they most likely didn't bother with multiclassing:
1. Additional work for UI-team
2. If you want to test classes and their functions and balance, its better to gain player experience with that class without mixing it up with other classes
3. It reduces bugs you have to deal with as player test it
4. It really doesn't pay off for levels 1-4 and most don't even multiclass until they passed level 5 due to multiattack and level 3 spells.
