Part of our development pace is that we are a small team (six engineers), bootstrapped, maintaining multiple products, and looking to do things in a way that fits with our vision for the products. We want to build products that are around for the long haul – Sublime Text has been around for 15 years. We'd rather focus on quality and performance than adding lots of features.
We are doing a big release because our current licensing scheme requires a "major version" release for paid updates. If we did a release once a month, they would all be trivial features, and wouldn't justify a major version bump.
For license holders, we've actually been shipping new dev builds every one to two weeks. However, since this is a major release, it has some very significant changes that need testing, refining and polishing. I don't think anyone in their right mind would ship a half-finished product and call it a major release, so we've been doing the work that shows it is a major release. The downside of bigger releases is that sometimes they end up dragging on a little longer than you want, and we'd rather uphold our vision for the product than have a release done a few months earlier.
As I mentioned in my post above, we've got some changes coming that will help address the "major version" issue and allow us to take on a faster release cycle. That said, I'm not sure I agree that new releases once a month are a good fit for the majority of users. We do, however, provide dev builds for users who do like seeing changes quickly.
We've got a super active group of some of the more prolific plugin developers that we interact with on a daily basis on our public Discord server. They definitely provide a lot of feedback and we make a point of listening to what the have to say.
The reality of it is that most open source developers wax and wane in their development work. The ones who stick with projects for years and years tend to either do open source work related to their day job, or are at least partially employed to work on the open source work. Others will get an itch, scratch it, share it, improve it and then be satisfied.