I concur it's a question of money and interest.
Pro races in the US can barely sustain enough interest to put together courses as tough as in Europe. E.g. the Tour de Georgia was classified as "HC" and had several very long days (182km, 176km 214km) but financially couldn't make the cut.
Many regions of the US offer the same population densities as in Europe, so I don't think that's a problem.
Of course, this is just the nature of sport. Different nations excel at, prefer, and/or are willing to invest significant resources into different sports. E.g. you're not going to find the same caliber of baseball players in France or Germany as you will in the US.
As to RAAM etc., those events are far too long, too spread out and lacking visible drama to gain much popularity.