Because it wasn't designed by PL researchers. It was designed by systems programmers who are used to C and just wanted a "better C". It was made popular because that happened within Google and they publicly gave it their backing so they wouldn't have to train new hirees on their new language.
Also its creator pulled a Molyneux and basically promissed journalists everything they asked about it. Not only would it be the perfect C++ replacement for all projects at Google, it would do systems and embedded programming and dozens of other things as well.
My first Go project (i think this was ~2014), i created a supervisorD clone as a school project (the coroutine/channel part of the languages were pretty much perfect for that).
After one week, i started calling Go: C+-. It felt like a superset of C with a lot of helpful tools, that kneecaped you each time you want to do something it's not meant to, like using memcpy. Why feel so much like C and not give you its most powerfull tool? (i was becoming pretty good with C memory management, pointer algorithms, and gcc at the time too, and not having those tools available to code/debug probably gave me a bad first impression).
The public backing by Google absolutely propelled Go into the spotlight, but Dart, also released by Google, hasn’t achieved anywhere near the same success. Considering how long ago Go was released, if the language didn't have its own merit, it would have fizzled out by now and failed to sustain its momentum or foster such a strong community.
Dart was never marketed (to my knowledge) as a general-purpose programming language. Go was marketed as the best thing since sliced bread, and especially as a "systems language", which it definitely isn't. It was also gaining popularity on HN at the same time Rust was gaining its initial wave of popularity (~2016-2017, around when I started reading HN), so the two were compared and written about a lot in a way that Dart never had the chance to since it never had a narrative foil.
In reality it turned out to be a worse C in many ways, because it has a GC and fat runtime (ruling it out for a huge chunk of what you might use C for) and lacks any kind of metaprogramming capability (yes, C macros are bad, but they're useful/necessary a lot of the time).
Regardless of their intention, it turned out to be a competitor to Java, not C.
