The only explanation for getting rid of dislike count that makes sense to me is to make big brands happy as their videos can't be made to look bad with a high dislike count and they can't be "dislike bombed" for things that may be unrelated to the content (eg the recent Activision-Blizzard scandals). Why? Because in every other way (and every way for the viewer), this makes Youtube worse. So it's a hell of a price to…
I am generally of the opinion that Youtube is up there with Facebook in responsibility for eroding democracy worldwide with their filter bubbles so while I get a lot of value from all the educational and entertaining content I consume on there, there is little love lost from me.
However, in this case I think there is another explanation which I think at least is the official one: dislikes were used to bully people. I don't know what the scale of the problem was but I can imagine it was a genuine problem and that they genuinely wanted to solve it. That's not to say that there weren't financial concerns as well, but I am willing to give them the benefit of doubt here.
By the way, I think the rationale was similar for hiding vote counts on this very site and I guess they concluded that it worked, given that they stuck with it.