You call it weak selective pressures, but another way of saying it is low fitness advantage. And we can see that because the programming language landscape is far from static, and newcomers do gain adoption very quickly every now and then.

In fact, when we look at the long list of languages that have become super-popular and even moderately popular - including languages that have grown only to later shrink rather quickly - say Fortran, COBOL, C, C++, JavaScript, Java, PHP, Python, Ruby, C#, Kotlin, Go, TypeScript, we see languages that are either more specific to some domains or more general, some reducing switching costs (TS, Kotlin) some not, but we do see that the adoption rate is proportional to the language's peak market share, and once the appropriate niche is there (think of a possibly new/changed environment in biological evolution) we see very fast adoption, as we'd expect to see from a significant fitness increase.

So given that many languages displace incumbents or find their own niches, and that the successful ones do it quickly, I think that the most reasonable assumption to start with when a language isn't displaying that is that its benefits just aren't large enough in the current environment(s). If the pace of your language's adoption is slow, then: 1. the first culprit to look for is the product-market fit of the language, and 2. it's a bad sign for the language's future prospects.

I guess it's possible for something with a real but low advantage to spread slowly and reach a large market share eventually, but I don't think it's ever happened in programming languages, and there's the obvious risk of something else with a bigger advantage getting your market in the meantime.