I think a lot of people have touched upon the major reasons here. In the non-H Sections, the forum software dying killed off interest in a lot of the older members that were there to talk to each other. In the H-Sections, the new rule changes have killed a lot of interest because of the higher posting standards.
Whether the rule changes are good or bad are a different debate that I don't really want to get into. However, I believe that people don't really "leave" the forum if they were only interested in the H-Section. The typical person who uses ULMF for h-content is mostly there to take and not give. The new rules have made it so that they have even less incentive to try to post. But they still benefit from the people who do take the time to produce content or information, so they don't actually leave; they just lurk.
As for the people who were going to post more, but thought against it in face of the new rules, I'm also willing to bet that a good 70% of them didn't actually read the rules and misunderstood the actual boundaries. I'm guessing they probably think at least one of the following:
- No links allowed at all
- No asking about translation downloads
- OPs need to be an epic novel length
- There are a million new additions/changes to the original rules and I can't be bothered to read all of them
Ignoring the last one, none of those were ever true. And yet because of some of the bans or warnings, people made the assumption that these must be the new rules because of what they saw, but never verified. Meanwhile, the gigantic stickied Rules thread is so imposing to look at, they don't even want to verify. But what they don't really realize is that there were really only two major changes to the rules: No asking for game links, and OPs have to follow a certain standard. The rest are addendums and clarifications to the rules because people keep on circumventing, misunderstanding, or otherwise needing things spelled out in no unclear terms.
The remaining 30% is the unfortunate part. They probably read the rules, and still made the decision that it isn't worth coming to the forum anyway. And to be honest, I don't really blame them. The rules were made to keep the low effort posting to a minimum, but because of how strictly we've enforced it, it's made the lives of those who've done nothing wrong harder. I'm not opposed to relaxing the standard, given that it's been more than a year since we first made the change and it looks like people have more or less adapted to it by now, but I also don't really like how many times I've had to change the rules already and think it shows indecision in the mod team.