There are several obvious reasons that possibly help to explain the current state of heavyweight boxing: The general lack of highly talented fighters (especially U.S. fighters or prospects, with the possible exception of Chris Areola) fails to create excitement; the mechanically rigid European style used by the Klitschkos and other foreign fighters bores spectators; the division's domination by foreign fighters disinterests many people in the U.S; the proliferation of meaningless titles (too many belts, like ibf, wba, wbc, nabf, wbo, etc) cheapens the title of “champion.”
Just to emphasize what I mean by lack of talent, consider the artistry and grace of Ali at his best; compare that example of talent with today's best heavyweights. Granted, using Ali is an extreme illustrative example (he's simply "The Greatest"), but look at some of the other great heavyweights of the past: Holyfield's rapid-fire, balanced, furious attacks, Tyson's blinding hand-speed and power, Joe Frasier's bob-and-weave tenacious attack style (with the greatest left-hook in heavyweight history), and George Foreman (maybe the heaviest puncher in the sport's history, who was great enough to be champ in the golden age of heavyweights, and then champ again 20 years later). None of the heavyweight fighters of this era could compete with the talents of the above individuals (and that list is in no way comprehensive, as many others could be added). If an underrated former champion like Larry Holmes were to fight today (and was in his prime), I believe he would clean out the division.