What are two reasons why it is difficult for unions to organzie the sort of retail clerks described in Nickel and Dimed, even in the face of the apparently poor employement terms and conditions that characterize their work?

From the text:

Once, long ago, unions had a place in American society, but they "no longer have much to offer workers," which is why people are leaving them "by the droves." Wal-Mart is booming; unions are declining: judge for yourself. But we are warned that "unions have been targeting WalMart for years." Why? For the dues money of course. Think of what you would lose with a union: first; your dues money, which could be $20 a month "and sometimes much more." Second, you would lose "your voice" because the union would insist on doing your talking for you. Finally, you might lose even your wages and benefits because they would all be "at risk on the bargaining table." You have to wonder - and I imagine some of my teenage fellow orientees may be doing so - why such fiends as these union organizers, such outright extortionists, are allowed to roam free in the land.