It's such an odd scene, their toys had to have still been on the shelves, right? I guess they didn't feel they had to kill them off though.

 

I think it's the same reason as with Dinobot's death: they wanted to make some space in case they had to introduce new characters, because having that many character models in the show proved kinda expensive for Mainframe.

At least, that's what I think.

 

I also think that they honeselty didn't know what else to do with the characters anymore. Tigatron's whole lone wolf/bot-of-the-wild/uncomfortable being in the base as a "proper" Maximal storyline had run its course and Airazor just never really found a niche or much of a personality after her introduction and that brief bit in The Trigger where she's kind of sarcastic/sassy. So they can clear the deck like TheUltimateBum notes and do it with characters that they didn't have much use for anymore.

I'd like to think they didn't axe them while already knowing that they were bringing them back as Tigerhawk. I prefer to believe that they thought this would be an interesting, unexpected, if somewhat rushed way to usher them out the door, then weeks later someone came back and said "you know that Tigerhawk toy? What if we put that character in the show, but made it out to be Tigatron and Airazor fused together? because hoooooooly balls was that ever a wasted opportunity of a character, and knowing that they intentionally jettisoned Tigatron and Airazor specifically so that they could turn around and reintroduce them as Tigerhawk would just make it worse.