My personal reasons as someone who has learned the language quite intensively but ultimately decided not to use it: (i) Vendor lock-in and too much future dependence on Adacore (other commercial Ada compilers do not count as alternatives to me because they are super-expensive), (ii) I can't always use the MGPL and would prefer MIT licenses of important Ada packages, (iii) the user community is split in a weird way between very professional aerospace and high integrity systems engineers and fairly dilettante hobbyists, but not many users in the space in-between those extremes, and (iv) not enough libraries for mundane tasks/operating system integration/GUIs.

I'm currently using Go. Although I would prefer Ada as a language, (iv) is in the end decisive for my tasks. If I used Ada I'd spend half of my time reinventing the wheel or interfacing with C libraries. I'm hoping to find a use for it in the future, though.