Too many US tennis fans still think they should have players on Agassi and Sampras' level. They don't even have players on Roddick and Blake level atm. Need to be realistic.
I doubt it's that; rather, many US fans seem insecure after the absence of Sampras, etc. These fans act as though American players are a direct representation of them. They seem overly concerned with how the country's tennis is presented to the rest of the world, especially in the absence of highly successful champions.
That's why when guys like Tipsy, Tsonga, Almagro, and co. clown out, discussion rarely transcends the player in question. Whenever an American player goofs up, it's treated as some greater implication toward the country's tennis as a whole. It's often an American or few ushering the most criticism.