There's two main reasons in my opinion.
The first reason is the easy one, that Mayweather hasn't made himself marketable because of his brushes with the law, abrasive personality, etc.

THe second reason is more of a US media issue and not a Mayweather issue. For decades, the mainstream US media and sporting culture has operated under the idea that boxing=heavyweights, and therefore stars must be heavyweight champs. The perception is that the lower weight classes just don't mean as much, which is completely idiotic when you consider that almost all the of the most talented fighters over the last 15 years or so have resided somewhere between 122-154.
Look over the last several decades, how many non-heavyweights have been big stars in the US??
Sugar Ray Leonard and Oscar De La Hoya are the exception, not the norm.

Pernell Whitaker was #1 PFP for 7-8 years, and he was never a household name. Bernard Hopkins didn't get endorsements. Even Roy Jones didn't really get much mainstream exposure for that matter. If someone walked up to 100 people on the street and asked them who Jermain Taylor is, I doubt more than 15-20 would know. If that.

Also, it's a little unfair to compare the notoriety of American fighters with those from the UK. The US is more than 5 times the size of the UK population-wise, it's easier to get lost in the shuffle in the US market than it is in the UK.