CuBit: A Gen­er­al-Pur­pose Oper­at­ing Sys­tem in SPARK/ Ada (2020)

I suspect it had a similar reputation as things like Fortran-- the principal use case defined it to such an extent that it prevents a breakout moment. An entire generation assumed "Ada is that weird bulletproof thing defense contractors and nobody else uses."

Languages like Rust have benefited from hobbyist showcases-- evangelists building familiar tools and demonstrating "it's more reliable and more understandable" but nobody was doing it for Ada.

I think the toolkit was proprietary and expensive until fairly recently, which I'm sure was a factor.