The reason it's deeper (right now) is because the meta is much more developed than Smash 4's. And I also disagree with your master-the-matchups-master-the-character statement: I feel like some characters will be able to be learned and, without too much thought, be able to bury a lower-tier character due to inherent advantages. (e.g. Rosalina + Luma can (atm) handle most matchups pretty well because they force the other player to conform to their playstyle)
I also think the healthiest thing for the competitive Smash 4 community to do right now is to stop trying to 1-up melee or defend itself to melee and to focus on developing it's own metagame.
This is all just in my own opinion though
