Comment by pjmlp

Mostly,

- Compiler prices

- The few UNIX vendors that supported it like Sun, it was extra on top of C and C++ compilers, why pay more when C and C++ were already in the box

- Hardware requirements

Still, there are around 7 Ada vendors around.

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/products_score/

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