Because Feathers on their bodies is Selectively Unfit
Contrary to some early theories about evolution, there is no such thing as "Use it or lose it". As far as speciation goes, you only gain what makes you more fit and loose what makes you less fit. Stuff that does not affect your survival generally just sticks around as is. That means that those feathers are either a genetically recent mutation that only ever covered part of the body in your birdmen's evolutionary history, or your birdmen have a specific reason to be more fit only having feathers on the back.
Early feathered dinosaurs may have had only partial coverage, but their feathers were short, fuzzy, and lacked structure because they had not been around long enough to really acquire all the features that we consider feathers today. Since your creatures seem to have long well formed feathers, this means your birdmen come from a long long line of feathered ancestors that evolved generation after generation to have those long, well organized structures. Because, their ancestors had feathers for so long, at some point natural selection would have gone for full body feather coverage because advantages in resisting the weather are typically going to be such a strong evolutionary prerogative. This leads to the conclusion, that these creatures are probably experiencing a specific evolutionary pressure that would cause them to lose their feathers rather than being an example of an early evolutionary state of feathers.
The closest example to this kind of evolution we have on Earth is Human hair. When humans learned to start wearing clothing, full body hair stopped being necessary, because we found a way to weather the elements without our hair, but places that proved to still benefit from hair even when we have clothing retained it such as our heads which are harder to cloth, and a greater source of heat loose than other areas. But, it was not actually clothing that made us lose our hair, but parasites. Fleas and Tics carry diseases that are harmful to humans, and hair gives them more places to hide on our bodies. So, evolving less hair to make us less attractive to parasites became more important to our survival than having enough hair to endure cold weather.
Likewise, your birdmen have probably retained their back feathers because they help with flight, stability, or some other survival important reason, but lost their other feathers because parasite resistance has for one reason or another become a stronger evolutionary pressure regarding their survival than hot or cold resistance. So, you could go the human route and say these creatures wair cloths, or you could just put them somewhere tropical but not too hot with very stable weather and lots of fleas and tics.