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?

Prestige - most likely. The fact that knowing/speaking French is considered sophisticated & elite in some circles. 

Agree that Spanish has way more native speakers & for all practical purposes it's a very useful language to learn