I notice among people I talk to more enthusiasm toward learning French than Spanish. This confuses me; my feeling is that Spanish unlocks far more countries and speakers than French does and so is a more natural first choice.

What do people think motivates this choice?

I think for much the same reason people love learning Japanese. We can probably ask, why do people learn a second language? I suspect it's not so utilitarian (i.e., being able to talk to the most humans) and more about impressing others exposed to that culture (so a soft power thing).