The way I see it, 6v6 singles has 3 eras of hoe the gameplay feels and functions
You have gens 1-3 with more blunt strategies, much more limited pokedex, and few item choices if not gen 1. A much more "solved" era of playing.
Then you have gens 4-6, which begin the complicated road of tricky strategies centered around more entry hazards, more elaborate setups, and many flexible item choices. You don't have a pokemon for every type and every role yet, but standout ones like Excadrill, Deoxys and Greninja began to powercreep older mons that may have been used less effectivley for the same role. I could say this centralized things, but compared to when you had to choose between Dragonite vs Snorlax, it isn't that limited.
Any point afterwards, the power creep got really prevolent. Tapus and Rilla forcing terrain play at the front of everyone's minds, pokemon like Dracovish or Houndstone with unique traits that seem very difficult to counter, and Pokemon with BST distributions like the Ultra Beasts, Paradoxes, or the gen 8 legends just took the power level to a new plane forcing players to rethink what is functionally viable. Even things that were banned to ubers caused ruckuses and made the divide between Ubers and AG this huge rift, M Rayquaza was not alone for this power level.
I say all this because the game felt blunt in early era, complex enough to have high tier big brained strategy in the middle, but fell back to a more unga bunga place. Even gen 7-8 Toxipex is pretty blunt for walling and stalling, many more examples of this around. The power creep and complex number of options has forced singles into a corner with how diverse it can be, and while smogon tiers can work at giving everything their time to shine, that only works as well as the playerbase for each of those modes are populating them.
With doubles, VGC in mind, you have much less concern for balancing because the nature of the mode makes survivability and in turn stalling much more difficult.
