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?

Romanticism, living in France, France marketing / cinema, historically french was the international language. The school / college my children go did not make Spanish a preferred language until 10 years ago, parents might be an influence, Spanish speaking countries might be considered.  lower class.

These book series are rather good Nuevo Avance Basico (Intermedio and Superior) https://amzn.eu/d/bj20gr4, Séneca (app) has great resources and watching Spanish programs with subtitles

Lack of learning resources (I found it really hard to find anything decent to teach / help my children over the years, moving from basics) My eldest just uses me now as tool whilst he finds his own ways. But anyway going from either language to the next will be "easy", or Portuguese / Italian