What evidence do you have? Have the networks tried to show such characters and failed? Or they simply failed to show such characters, claiming their usual bs (this is what the audience want) as an excuse. This is a bona fide question because I simply do not know, since I do not watch TV except PBS, the weather channel, and the Simpsons.
I doubt, however, that characters like Schwartz would do poorly on TV series. I also think that foreign characters in certain roles (e.g. immigrants being Americanized) might do well, for example a series "tracking" a Russian immigrant from "getting off the boat" through working his way up in the US society. Of course, the character would have to be physically attractive (preferably young and male), showing certain character traits (being honest and somewhat naïve, but also hard-working, entrepreneurial, and intelligent, with a good but not too sophisticated sense of humor), and engaged in situations that show mostly good and funny things about the US society i.e. "everyone can make it here, despite initial difficulties (s0me of which would serve as a comic relief). That also implies that most US-ers should be essentially good characters or turn out to be such at the end of the episode, and the only bad characters in such series be either other immigrants or members of generally despised categories, like drug dealers, lawyers, greedy bosses, or politicians.
Wojtek