Why does nothing stop Tryndamere from ulting but even a root stops Ekko's ult?

Trynd used to be unable to use his ult during CC. It's duration was significantly longer, though.

Well, first, Tryn does have a small cast time for his ult.

Other than that, the reason is because Ekko has to teleport to wherever his... err, holograph is. Unlike in Smite, you cannot use mobility skills while rooted. So Ekko can't ult if he's rooted.

Because Trynd is a terribly designed champion that needs the most bullshit ability in the game to be even remotely viable. If his ult couldn't be activated while CC'd he'd mostly likely be terrible because he needs to have absolutely no counterplay to work.

Ekko is thankfully designed to have actually functioning strengths and therefore his ult doesn't need weird exceptions like Trynd's.

Because Ekko's is a dash even if you use it to go to the same place. I'm unsure why nothing stops Trynd's ult, always wondered that myself. I assume because it'd be waaaaaaaay too easy to beat trynd if that was the case, but it's pretty inconsistent considering Kayle can't cast hers during stuns or silences.

It's like his own QSS. So no matter how screwed he may be, just as long he has a working R button and decent reaction time, he can mash it and (hopefully) get out alive.

I think Tryndamere's ult is like Gangplank's oranges, to an extent. GP can eat oranges when CCed. Tryndamere's ult seems to be coded similarly.

Because he's always been able to use his ultimate while crowd controlled?

Horribly designed cancer like Tryndamere doesn't follow the common balance rules