It starts and ends with the fascination of the heavyweight division in America.
The hw division is considered to be boring and lame with no true exciting fighter in that division. Yes the Klitscho brother are dominant, but not exciting, it says something when HBO and Showtime do not want to air the #1 and #3 fighter duking it out.

In fact there will be a major interest in boxing again in America when an exciting HW and dominant champion is there, he doesn't even have to be American since we live in an more tolerant and globalised society.

The HW division has always been considered to be the life line of the sport for a reason. The 1 true HW champ is considered to be the face of boxing to Americans. And America is considered to be the mecca of boxing, in terms of money, coverage, etc.

Boxing was always a popular sport in America during the days of Dempsey, Louis, Ali, Tyson, Holyfield, and then when Lennox Lewis was the reigning champ, most Americans got turned off and it has nothing to do with him not being American, but boring and same thing is applying to the Klit Brothers.