The language not having one clear thing it excels at, surely makes marketing it more difficult, so Nim would have needed more effort than normal to properly explain its philosophy to the market.
Community building and governance is also critical. Nim has a fork called Nimskull that was created because of disagreements in the core team. I don't have any insight on what happened there, but regardless having part of your core team splinter off into a separate hard fork is not good for the growth of the project. This is a HN thread from that time: https://news.ycombinator.com/item?id=32021299
Lastly, it's also a matter of luck. Sometimes you just happen to be working on something that becomes more relevant all of a sudden in a way that you couldn't predict, and some other times this doesn't happen. Zig for example became more relevant once M1 Macs came out because now all of a sudden arm64 is not a "theoretical" target anymore. Now the laptop that you want to use is arm, and so cross-compilation suddenly becomes a real thing to worry about.