I was kinda generalizing the entire base game in terms of simplicity since they are a lot less ambitious, especially considering a lot of the things they could do.
Now that I think about it, this is probably where the idea of DLC treatment comes from, where they're given a lot more options in comparison to the base game cast. It's also kinda odd IMO that characters that are way more ambitious like Steve are usually held off for DLC, despite having unprecedented mechanics that feasibly would have been easier to incorporate in base game, considering they'd have been able to alter the pre-existing engine and account for stuff like that. I guess it could be argued that maybe these characters could never have been in base game but Steve isn't really the only example. It definitely applies to most of pass 1 and definitely all of pass 2. This is me discounting other potential hangups BTW like timing etc before anyone starts with that lol. It even happened back in Smash 4 DLC. I imagine it's more so the circumstances that means that other characters were prioritized at the time but I guess maybe if not, it could be that they want the base game characters to be scaled back so they can appeal to the more casual demographic who are ultimately the majority. Food for thought.
The thing with Base game is that the entirety of the game is being developed at the same time the Base game Characters are being developed. The more complexities a character has, the more time has to be put into that character
One major thing is the Balancing. Every character needs to be balanced against the entirety of the Cast, and Characters with Complex Mechanics not only need their base Moveset to be balanced, but also their Gimmick against the rest of the cast. that takes alot of time. This is probably the biggest factor as to why Echoes or Clones are so easy to make in comparison to Unique Fighters, and probably why Certain characters (Doc) aren't considered echoes since they have enough differences that they need to be balanced Differently from their base
Plus, there's just also the thing of figuring out how to implemented Certain Abilities and such. Implementing something lie Ryu or Bayonetta in the base game would be much more challenging since these 2 had systems that were never coded in the Smash game, to to develop and code those systems, balance those systems, while at the same time Adding the other Base Roster characters, would be madness
And another thing is that Base Roster tends to lean towards 1st Party Nintendo Newcomers, while DLC Lean towards 3rd Party Newcomers. Because 3rd party characters requires that the owners be on board with Development process, that's gonna increase the Complexity of Developing the character since everything needs to be approved with the Owner. This becomes even harder with Western Characters due to the Language barrier, Timezones, etc. There's also the Licensing fees and such, which I bet Nintendo would rather take the hit on DLC, than on the Base game
DLC allows the characters to be as Extreme as they are because the Base Game is already made with the Systems and Balance set in place. Instead of having to develop some of the challenging systems DLC characters have when they are Making the base of the whole game, they can just Add onto and expand an Already made game with New systems, which is probably much easier to do
Also of course since people are paying Extra for DLC Characters, the Dev team has to go all out on the characters to make them more enticing. The Base game characters are sold with the Whole game and such