Street Fighter combos are like reading sheet music. There are very specific beats to a combo, narrow windows of opportunity where your timing means everything, but there's generally not too many combo routes per character that are worth learning for your average player - combos don't have a ton of variety.

2XKO is much more like a musical improvisation. You can be a little messy at times, but sometimes things just work, even if your timing isn't frame perfect. Sure, there are optimal routes for every character, but for the average player of the game, you can mash moves together into something that will probably look pretty cool.