They're not selling well for many reasons. The full purchase price vs subsidized price vs current T-mobile customer options are all confusing. Customers also don't have the chance to "try before they buy" and test the phone out in store, which I think prevents a lot of sales.
Plus the marketing just isn't on par with other devices. Look at the Motorola Droid, a comparable Android device. It's selling very well, perhaps due in part to Verizon's and Motorola's heavy marketing of the device (TV, radio, billboards, online ads, etc).