This will probably sound like a smartalek answer, but I guess it just doesn't make good business sense right now.
I think the design and construction costs of new condo buildings, like the really nice one proposed on Randolph, preclude affordable units. The city would probably have to provide some incentives to bring the profit margin up for the developers.
Renovating existing older buildings are a different story though. I think the building behind 301 East is low income, right? That building could probably be renovated into affordable units. Same with the old hotel downtown (name escapes me).
I think there is just a general lack of desirable, affordable residential real estate in the Huntsville metro area period.