Two things come into play, off the top of my head. First and foremost is supplier capabilities and/or projecting demand to match supplier capabilities. There are several items unique to the Sonic RS, the more complex such as the seats requiring a greater lead time/manufacturing time.
The second is CAFE. Most specialty vehicles (frankly I can not think of an exception so should be all) carry lower epa mpg averages than the rest of the vehicle line. Based on the above referenced mpg report, this may not be as much of a consideration as my first stated reason.
Chevy does not intentionally limit production to make a car more valuable in the future. They will make as many of a model as parts available will allow along with dealers willingness to buy. Chevy would have little concern if every Sonic RS assembled sat on the dealers lot as they get there money whether the dealer sells the car or does not.
This is my answer to the question, others with greater knowledge should add or negate. I am certain I am missing something.
Laborsmith