Given that there are still 7 vendors selling Ada compilers I always found that argument a bit disingenuous.
https://www.adacore.com/
https://www.ghs.com/products/ada_optimizing_compilers.html
https://www.ptc.com/en/products/developer-tools/apexada
https://www.ddci.com/solutions/products/ddci-developer-suite...
http://www.irvine.com/tech.html
http://www.ocsystems.com/w/index.php/OCS:PowerAda
http://www.rrsoftware.com/html/prodinf/janus95/j-ada95.htm
What is true, is that those vendors, and many others, like UNIX vendors that used to have Ada compilers like Sun, paying for Ada compilers was extra, while C and C++ were already there on the UNIX developers SKU (a tradition that Sun started, having various UNIX SKUs).
So schools and many folks found easier to just buy a C or C++ compiler, than an Ada one, with its price tags.
Something that has helped Ada is the great work done by Ada Core, even if a few love hating them. They are the major sponsor for ISO work, and spreading Ada knowledge on the open source community.