What are the factors that contributed to the rise in popularity of Jamstack and the sustained growth of its adoption?

There are a lot of factors. I'm not sure that there was one obvious thing for me that made it blow up. But I do think that Netlify was a big part of it. They coined the term Jamstack, promoted it as a concept and proved it out. They spent a lot of effort to create an ecosystem that could compete with Wordpress. A big moment for the Jamstack ecosystem was Netlify scoring Smashing Magazine on their platform and showing that a big production content site could use this tooling. So I think a lot of it was them pushing the industry forward.

A few other enabling factors were static site generators becoming more popular, especially GitHub's Jekyll generator. The GitHub ecosystem in general becoming the default way to create and version code. Around that time Github’s API allowed Continuous Integration tools to hook into the workflow and run automations whenever code was pushed. So I think static site generators, GitHub and CI were important enabling factors too. 

To give Netlify even more credit, I think that the build hook was a small but essential innovation that unlocked Jamstack as a workflow that could compete with Wordpress. Before the build hook, you needed a developer to compile and deploy website changes manually. If you had a non-technical content author that was creating markdown files, getting that person to commit to a Git repository was a big hurdle.

Having build hooks allowed a third-party API to trigger the rebuild. That opened the door for headless CMSs to offer a non-technical interface for writers to produce content. The CMS could automatically rebuild the website via build hooks whenever the content changed. That sorta made the whole thing work.