It's not mysteriously absent. It will be returning in one of the last two MCWs.
Every single other 3rd Party costume from Smash 4 has returned, and since all 11 characters were decided on in 2018, so was the decision to re-release all those costumes for more money for all parties. However, as I've said a hundred times in the last month or so, his costume means nothing for his chances one way or the other.
As for Gil and Heihachi, their costumes aren't killers either, but rather their situations. Gil is from a franchise that isn't really a thing anymore if we are being honest with ourselves, and Heihachi was attempted but ultimately it was deemed not possible to accurately and properly do Tekken justice in Smash due to the complex nature of Tekken versus the innate simplicity of Smash. See, with games like Street Fighter and Fatal Fury/King of Fighters, characters have iconic moves they always have so grabbing moves to pick is super easy. They are also 2D Fighters and have less variety of inputs, so they are much easier to realize. Go look up a Tekken Movelist in your spare time and look at how complex just some of the directional inputs are, then realize many moves rely on inputting them during a rising-from-crouch animation or during a side-step animation or cause them to crouch/sidestep and now you get a glimpse on why it wasn't possible to do for Smash 4.
The question for Heihachi is if Sakurai found a way to make it work or not, which I doubt he did. Tekken is just too far removed from Smash in base design to make it happen without taking out tons of elements that make Tekken what it is, but you really never know. On top of that, Tales Series is very popular and both Yuri and Lloyd are much more requested, so it really makes more sense to go that route.