Your productivity in any language is directly proportional to your time and effort investment in it. It's in your best interests to pick the language that's likely to thrive and spend time learning its ins and outs. On the flip side, betting on a horse that doesn't win could mean the loss of months or years of effort. This is why people evangelize the platforms they're invested in - convincing other people to join improves the health of the platform, increasing their return on investment. More adoption => more libraries => more projects => more jobs => more adoption. A virtuous cycle.

This evangelizing can sometimes become contentious if others perceive it as an attack on their platform. People defend their language mostly because they don't want to see it lose popularity. If it did, their language's viability is threatened and their investment is in jeopardy.

It's also partly because they've spent so working with this language that it's become a part of their identity. A critique of the tool is perceived as an attack on the person.