Generics were around for some time now, and people don't seem too keen on using them for anything aside from data structure specialisation, because frankly dynamic dispatch via interfaces hits such a sweet spot where you get a lot done with a very minimal overhead. The added syntactic complexity is very rarely justified, and containers is perhaps one of those cases where it is the case. Otherwise, from what I observed— generics in Go were massively over-hyped, but failed to gain the traction that was initially expected from it. I feel like there's a very good reason for that.

Generics as a feature is almost invisible when it exists and is done well, but is a huge pain in the ass if it doesn't. It is mostly needed for libraries, so your average dev won't write them often, but there it is invaluable.

Yet even Objective-C, eventually got generics in 2015, despite protocols and categories.

The good reason is the target demographics as Rob Pikes puts it, people that don't care about learning what programming language can be like.