Basically lack of polish. Nim's compiler just keeps evolving around unfinished and unpolished ideas never reaching a momentum.

It's a research project of its BDFL and he is alienating the most senior developers and contributors to Nim's ecosystems (ie Zedeus).

Go has Google behind it. It's a much more established language with far more mature packages and a much bigger ecosystem. If you're using Nim then you're relying on few libraries/tools written by hobbyists who don't have time to ensure they work well, or even more likely you're having to write these yourself.

But the bigger problem is the culture of the Nim community and its leadership. I made a hard decision to stop spending my free time contributing to it a few years ago now and I'm glad I did.

More stable, commonly supported platform I presume. You don't want to be on a snowflake platform for a long running project, it only causes endless problems.