Personally I think its in your blood...or not.
I agree with the numbers being posted...many things are contributing factors...how about the lack of need of a passport in order to experience many types of cultures and climates, Some of my friends say if I want blue water and a nice beach..I can have that without a passport and a long plane ride...If I want to experience culture I'll go outside of my neiborhood...all kinds of people everywhere...so why leave and go into an area you have no rights??
Now I'm not one of those people who likes to stay home, so I enjoy the adventure, I won't travel to the same place multiple times a year because for me I want the experience of something new. Now out of the other 75% I would be willing to wager that economics plays a role...beside you can rule out very high percentage of minoritys who rarely travel...regardless.
Mindset also effects the rest...if you haven't noticed we are not the most popular group of bastards
Oh well such is our lot in life.