If anything SC is technically 'more' complicated (for one, it has a color picker with saturation values unlike SFxT's preset colors), but its a different beast due to how the models are handled and rendered. In modern SC and Tekkens, the character model is basically modeled 'naked' or with a base layer of clothing and the reason you can modify the parts like that is that the clothing models and additional 'parts' are usually completely separate from the characters models. Smash characters are usually modelled as a full piece and loaded as such as well. which is why each 'colour' is a separate model. You
could restrict which parts are colourable but that doesn't really make it easier on the dev side for this hypothetical editor.
The stage thing is programming wise to accommodate a character's moves and what values to return which is pretty different from remaking basically all the character textures, programming in a thing to discern which colours are allocated to what in the texture files (some textures are also things like logos , patterns, and images so you cant just recolour the whole texture) and making an in-engine color handler. Not saying it's impossible, just that it's a definite undertaking and basically a complete overhaul of the engine and how it handles rendering textures, so if they were ever planning to do this they'd have to be developing with the intent to do this and accommodate this from the start.
As a further example, here's an image of one of Tekken 7's outfit textures, which similarly to SC is a FG built with recolouring in mind. See how everything is uniformly monochrome, kind of organized into something like a grid?
As a comparison, here's an example of a smash texture:
Not a fighter, and from the prev game but I think he makes for a good example. Note the more liberal use of gradients, patterns and designs, more strewn around layout, and all sorts of things that would make palette mapping difficult. To work this into resembling the above for machine colour mapping would basically require remaking it and then reprogramming the engine to tell it how to apply colour and texture onto it.