It does appear they have an error on the specs for the 45....the 40HST and 40HST Cabbed have the same PTO power, as do the 50HST and 50HST cab models. It makes no sense that the 45HST cabbed would have 2hp more PTO power than the open station.
Regardless, if you take a DK40SE HST Cab, and turn on the AC, you're down to working with something like 26-27hp, which is pretty low for the size machine it is. With your 45, it's still down to probably right around 30-31hp in the same configuration, which could be marginal if you size your implements for the amount of PTO power you have without the AC on....and you know that's how people will likely do it.
I'm not saying other factors might have come into play, but I can certainly see why a dealer might not want to stock a 40 or 45hp tractor that loses a significant amount of power to AC loss...they can always get one pretty easily if somebody wants one. My dealer just said they don't recommend less than the 50 in a cabbed model....that's all. If somebody wants one, they'll gladly sell them one.