Is it because of runtime speeds (the slowness) that prevents microkernel operating systems from being more prevalent? Or is it just because Linux is good enough, and it became popular and free?
There is a speed impact but it's mostly negligible, especially on modern hardware. The main reason is that building an entire operating system requires enormous investment beyond what most companies are willing to spend. But Google seems to be willing. And even if you succeed, people will inevitably chase you on Linux app compatibility.
Some well known open-source examples are seL4 and RedoxOS.
Agreed. More services does mean more context switching. The most pure microkernels will have many, and the more practical ones may consolidate some services to reduce context switches. Also, some types of IPC can have a very lightweight context switch; they typically transfer only a few words of data, but this is usually mitigated by shared memory.
Some well known open-source examples are seL4 and RedoxOS.