(I can't speak for the Japanese UTAU community tho)
I think the thing was, back in 2009 - 2010 or so, UTAU had such a much smaller community that I think is why people sort of just knew everyone more? As it grew, everyone had their own little groups.
Those groups grew off each other and people bounced back ideas, collaborated more, and shared their work and passed it around. Sometimes, the person would get so big you'd be like "Oh yeah that's ___ who has this UTAU named ____"
I really don't know what it was that caused it to die off, but my biggest guess is that as those people got older (like, when I started out with it, I was 13. I'm 20 years old now, so a lot of time with it has passed) and once you get older, you go through college, jobs, etc. the like, you get a lot of responsibility.
Making a voicebank can be tough on some people, and for others UTAU just started to drift apart from them.
For me, it kinda just stopped being rewarding. I love my UTAUs, I still love them a lot, and I've almost converted them into simply OCs now although they still have voice banks. The thing is, with the community dying and most of my friends moving on, it felt like my work with UTAU was pointless. I still loved making the art since art is something I did already, but I didn't feel a point in doing covers and what not. So I stopped but that's just me.
UTAU used to kind of feel like UTAUs had a personality through their voices, and I'm sure they do, but I just can't keep up anymore bc I'm older I guess.
I feel like it definitely had it's time, and to be honest with you I kind of miss it too. Buut as Kiyoteru said above, I'm pretty sure UTAU does have its other places where it's more active. Maybe as a whole it's not how it used to be is all, but I'm sure it'll still be there.
On the topic of how to revive it, I wouldn't know where to start really other than as you said, starting more tutorials? I think that'd be wonderful. I remember back then there would be a bunch of tutorials, but I still stumbled around.
I think one thing to keep in mind is that in this time, CV style of recording is like good and dead so I hear. That was a really simple way to make an UTAU which felt like very doable for someone who's never done it before. VCV seems to be the new standard now, and if people really get into UTAU these days, they do voice banks with more than one set of recordings.
EDIT: I think KNΞMΛTCS was pretty spot on. Along with what I said before on UTAU no longer feeling rewarding, it did start to get boring.
I'm sorry if I sounded so doom and gloom the majority of this post LOL It's just, everytime someone brings up on whether or not UTAU is dead, I get all nostalgic I guess. I've been seeing this same question since like 2014 - 2015 too.