I think it's because we are wired that attention is it's own currency nowadays. And it's also true. Even if there's no paid product, you get strength in numbers. If you depend on an open source library, it's usually better for you if others depend on it too.
Certainly an element of that but there are also cases where the superior product "lost" to the inferior product because the inferior one was better marketed.
So doing some level of promotion becomes necessary if you want users even when you have the better product - the superior product speaks for itself doesn't often apply any more.
And programming languages are in the lower end of quality actually impacting decisions. People are incredibly resistant to changes there, and just can't evaluate competing options at the same time.
