The animal crossing community has grown larger over the years, and with more people joining the fandom, we are going to be seeing more opinions from both sides of the spectrum.
If I can share an opinion, (and this doesn't apply to ALL time travelers) I think a large part of the problem is no one has patience anymore these days. They want things now. They want things completely their way. So rather than playing the game at a more natural pace, a little bit of progress each day, people chose to time travel. Aaaand they got everything unlocked in a matter of days. And then they ended up sitting around with nothing left to do, and were unhappy about that. Some time travelers understand and purposely do this because they are happier to play at a faster pace, and that's fine! But I believe there's quite a few others who jumped into time travel without really thinking about the consequences of this, and were left unhappy because they "ate the whole cake in one day instead of enjoying a little piece each day to make it last longer" >.< That's one source of the complaints.
Another thing that people have a hard time adjusting to is that they became attached to certain things in new leaf and feel unhappy at not having all of those things available at this time. Whether they are left out entirely or will be added at a later date, no one can say. I hope people can realize that this isn't New Leaf. It's New Horizons. An entirely different game. It might not have everything that new leaf had, but I don't think we honestly got a horrible animal crossing game yet. We're going to have plenty of content, even if it's not the same content.