However both were born alongside UNIX and that helped C++ to be quickly adopted by all major C compiler vendors, whereas Ada was always something extra to pay on top.

When targeting UNIX, with C and C++ compilers on the box, who is going to pay extra for the Ada compiler unless required to do so?

I was thinking more in the 90s where GNU already had a freely available C compiler, but GNAT didn't get a free version until the late 90s, and even then it was built on a fork of gcc you had to download separately until like 2000. It was just a lot of work to get up and running, as opposed to the bundling of C/C++ compilers with Linux that was common, as you say. The initial C compatibility helped C++ a lot too.

Still same rule applies, when a UNIX shop paid for UNIX developer tooling, usually languages like Ada and Modula-2 weren't in the box, you needed to pay extra.