Historically, the operating system was largely used as a research platform for writing papers and theses [2]. Unfortunately, research code isn't production-grade code, so despite a lot of effort done to mainline some of it, not all of it was merged. Also, while there was a steady supply of new students to work on MINIX3, there was also a steady turnover because students who finish their studies generally move on to new things.
With the looming retirement of Andrew Tanenbaum in 2014, the project pivoted focus to embedded systems, due to the technical strengths and weaknesses of the system at that time. The ARM port was done around that time as part of it to leverage small board computers. Eventually, the funding dried up and the project slowed to a standstill, largely I believe due to an unsustainable maintenance burden caused by accumulated tech debt over decades that the remaining development manpower (all contributors working on their free time, including myself) couldn't address.
MINIX3 still has some unique cool stuff to this day like seamless live updates of system components, but for me it's too shackled by its past to have a future. It'd be a lot of work to bring it up to modern standards and personally by the time I'd be done with that I think ought to be done, it wouldn't look like MINIX anymore.