Zenurik is definitely an appealing focus tree for obvious reasons but I think przemo887 and Steel Rook does a good job hitting on some of the other problems here. A cursory glance online has pretty much everyone telling everyone to take Zenurik as their first tree and given how rough DE has made focus progression without specialization, a lot of people are going to be stuck with their starting tree for a long time.
Second, the Operator system itself is pretty niche right now. It's an important part Eidolon hunting, but that's a self contained ecosystem and outside of that and one and done story quests they aren't used all that much. Combine that with gear upgrades being part of the aforementioned Eidolon Hunt ecosystem and it means a lot of people aren't going to worry about it much. That means that 8/10 upgrades per tree aren't going to be relevant for those players either.
That's not to say Zenurik isn't good and some of the other trees aren't pretty bad, but between slow progression, tedious acquisition of upgrades and how much of a walled garden Operator content is, it shouldn't be surprising that a lot of players just throw their first few points into the most ubiquitously useful tree and then never look back.
Anecdotally at least I've wanted to try some of the other trees, only to remember that I'd need to grind lenses, upgrade those lenses, then engage in some cheesy-feeling tactics tangential to the core game in order to efficiently level those things up over the course of a couple weeks and... why would I want to bother with that?
I feel like even without any balance passes if the focus/operator system didn't feel so hostile and disconnected from the rest of Warframe there'd be more variety on that ground alone.