Because for Dark Decade, WCAX, and OOO Tatoba, they're all characters who appeared for less than 3 minutes in the whole series, and they're pretty much a rip-off of the original. Meteor Fusion might be another possibility, then it will be a rip-off of Cosmic States and including Meteor's moves. But it'll be hard to do a super finisher for him since they probably wouldn't be bothered to make a super for secret characters.
I'll explain myself.
Dark Decade never appeared in the series, it was a Climax Heroes only character. In the original CH, Decade was the baddass character, having a second Decade to even the match made perfect sense.
WCAX, appeared for less than 3 minutes but aside from being a plot driver he was a separate "configuration" than W (the main body was Terui's instead of Shotarou's). Dark Decade was not necessary so CAX made sense because it was pretty much a nerfed villain.
OOO Purple Eyes was also a plot driver, you can argue that it is not an official Combo and the fact you can only select him while highlighting OOO makes him more of a Gauge than an actual character (just like Birth Prototype is Birth's Tag Gauge). Also, in the series, OOO used a lot the Medajyaliber + Medagaburyuu combo. It made sense to put that in as well.
Libra-Fourze is nothing, just a form to trick people, he can't even use switches and, honestly, there is no point for him to be in the game since there is not enough stuff he can do.
Meteor Fusion is still Gentarou with the Fusion Switch, while it would be easier to program him it wouldn't make any sense to make him a separate character like WCAX and Dark Decade. Of course, he could work like Purple Eyes, but still... Fourze Meteor Fusion States is a good guy, it would be just awkward IMHO, same goes for RocketDrill States.
That being said, I have absolutely no idea which one will be the "secret character" or that if we will really get one this time and if it is really gonna be a Fourze Rider (Poseidon?! Core (that is getting a LOT of attention lately with RG2 and Gambaride)?!).
