Highly constrained OSes and languages that do so in order to minimize attack surface tend to be challenging to work in. As a result they constrain productivity which increases time to market and the folks who got something out, even insecure, would "win" the market. It was a sad thing to see happen.
The other aspect was that IPC via message passing is a very natural way to program.
That it has never taken off is more evidence that there's no money in securing software, just cleaning up the mess insecure software leaves behind.
I am under the impression that the people behind seL4 have managed to successfully commercialize earlier other versions of L4 before seL4 was created.
Anyway, even if we grant the premise that seL4 has not taken off, that does not seem to justify saying that there is no money in securing software.
I might hazard to say that (in my opinion) no OS written in a memory unsafe language is secure by design.
EDIT: bzlg. SYSGO GmBH