there are many reasons, such as the ones already mentioned. also, unless i'm mistaken, savate is less popular in it's home country than muay thai is in it's own. this means that there are more indigenious fighters out there representing their style.
also, savate's reliance on kicking with the boot & prohibitions against kicking with the shin prevent it from performing well against other styles, regardless of how effective it is. also, if i can make a huge genralization, there is a tendency among the french to refuse to "market" any aspect of their culture; the prevailing thought seems to be that what they have is so great that others should come to them. you can see this in french language, philosophy, & art. the french have a lot to offer, including savate, but most people will gravitate towards what is more accessible.
jf