alt Hacker NewsGNAT has existed since at least the mid-90s, and in that time period plenty of companies used non-OSS compilers.
In that era, the largest blocker for Ada was it ws viewed as having a lot of overhead for things that weren't generally seen as useful (safety guarantees). The reputation was it only mattered if you were working on military stuff, etc.
True, but at that time it was already too late. C/C++ had won.
Moreover, for a very long time GNAT had been quite difficult to build, configure and coexist with other gcc-based compilers, far more difficult than building and configuring the tool chain for any other programming language. (i.e. you could fail to get a working environment, without any easy way to discover what went wrong, which never happened with any other programming language supported by gcc)
I have no idea which was the reason for this, because whichever was the reason it had nothing to do with any intrinsic property of the language.
I do not remember when it has finally become easy to use Ada with gcc, but this might have happened only a decade ago, or even more recently.