The same argument can be made for most languages, and also as a serious question, why wouldn't they be interested in learning Celtic languages?
Most people aren't interested in languages, period - they won't learn one unless they're forced to for some reason.
So given that, why is it surprising that when someone is actually motivated enough to learn a language, then don't necessarily choose one with the most general "utility"? As I look at it, people who learn languages for fun are freaks to begin with. Is it a surprise that they often end up choosing freakish languages?
I'd been studying Welsh on my own for a couple years before I went to college and met my first honest-to-goodness Welsh-speaker. He was a learner like me, but he'd had the opportunity to go to the Llŷn Peninsula and be immersed in the language. "Don't try to explain why you want to learn Welsh," he told me.
Someone once told me that if you really want to learn a language, it's not enough to hang out with it. You have to marry
it. By the same token, explaining why you love a language enough to want to learn it is like explaining why you love a person enough to want to marry them. Which is to say it defies rational explanation altogether.