BeOS, Symbian, L4, Genode, Mac OS X drivers, and big parts of Windows are all examples of C++ use in OS development.
They had zero vendors willing to put their OS on a box and resell it because MS threatened the vendors with pulling their right to sell Windows if they did. This wasn't particular to Be, but to any other OS.
However regarding Microsoft, I actually think that vendors were as guilty as Microsoft.
They could have chosen not to take Microsoft's discount and try to sell alternative OSes, even if it meant having to face a few challenges.
The one that takes is as guilty as the one that gives.
I did a quick search and came up with Quora answer given here: https://www.quora.com/Why-was-the-BeOS-dropped
but this doesn't jive with my memory. I will have to see if I can find an old email or maybe dig further on the net. I certainly don’t want to be rewriting history.
I remember quite a few small shops trying to put a fight selling other kinds of computers, they might have lost in the end, but they tried to walk a different path.