why should I even care if language X is on the rise, or decline, if it's user base or kloc is huge or miniscule?

There are different reasons for different people.

Job seekers want to know what to learn so that they can be employed. Your favorite language may be great and all, but it won't matter if no one will hire you to write in it.

Employers want to know how to implement their next project. They want a language known by many programmers so that it's easy to find people to work on the project now and in the future.

Hobbyists want to know where they can get support. They want a language with a strong community and good documentation so they can teach themselves and get help when they're stuck.

There are still other reasons for other people, probably.

Well said. You got my vote ;)

You could also add:

Employees who need to make the case to their possibly less informed superior why they continue to work in such an obsolete, "yesterdays news" language.

A blog among millions.

why should I even care if language X is on the rise, or decline, if it's user base or kloc is huge or miniscule?

I think the broader question is, why do people care whether languages are "popular", or that its user base may be slipping? Why is this such a frequent topic of discussion? I can't say why you or anyone should care, but I do know that many people seem to.