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?

As a speaker of both language: 

I get better reactions from strangers when I mention le français, although Spanish has been ten times more practical

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As a speaker of both languages, that's been my experience as well.