D&D 5E Optimal Multiclassing

[Eejit]

I really like the choices 5E forces for multiclassing. I was considering making a Barbarian -> Rogue (medium armor/shield/Str rapier sneak attacker) multiclass but it's an interesting choice deciding when to swap.
At 1 with a single rage and the armor proficiencies? At 2 for Reckless attack and Danger Sense? Barbarian 3 with the sweet first subclass ability? 4 for the ABI? 5 for the second attack and speed? 6 for the second subclass benefit? 7 for initiative advantage? It just gets ridiculous

If all classes are relatively balanced, why is Ftr Str OR Dex, Mnk Dex AND Wis, Pal Str AND Cha, Rgr Dex AND Wis, and all other classes are just one Stat?
Ftr being 'either/or' implies it's easier to MC into.
Mnk, Pal, Rgr being 'and' implies it's harder to MC into those classes.

Rangers, Paladins and Monks are all semi-spellcasters with save DCs based on the second required stat.

Fighters, Rogues and Barbarians are not (with the exception of EK and AT).