It has been discussed earlier, and elsewhere; but I am led to believe that CVT and Jersey are not great airports to operate the 737-800 out of/into; so by basing the older a/c at CVT TFly can retain the CVT - Jersey route!
A friend who lives on the Island said this was reported on local radio (which is normally quite informative) as being the case; hence a lot of TFly's other bases having Jersey axed!!
Shame though.