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?

This is location-dependent

If you're North American, Spanish is either a necessity or at least a part of daily life so, while French is exotic

If you're European, French is much more relevant: it's spoken in 5 EU countries plus most of North Africa (big tourist destination)