Now, this is just what I read from a CFI Candidate taking his checkride at the Ft. Worth Alliance FSDO-
I guess he asked 3/4 inspectors why Spinks and Prairie were G towers, when the have the same dimensions, entry requirement and night weather minimums, because they could easily be Class D's. Unfourtunately not one inspector knew the answer or knew anyone who did. 
Supposedly- To be a Class D tower the airport needs advanced weather reporting equipment which would eventually lead to an ATIS. As where Arlington, Spinks, and Prairie all have ASOS3.
Again, if anyone thinks this is wrong, let me know.