Sleepwear has gone through several iterations these past hundred years. A century ago, the standard was the pullover muslin nightshirt, which kept the wearer warm in an era without central heating - or rather it did so long as his tossing and turning did not pull it above his ears during sleep. That latter problem and better heating helped convert the majority of men to the coat and trouser pajama in the 1920s. The pajama's suitability for a lounge-around suit a la Hugh Hefner helped keep it the sleepwear standard for decades.
Relatively recently, changing retail economics has encouraged a shift from pajamas to shorts and tee shirts. Where pajamas required a store to stock a considerable inventory, shorts and a tee sold as separates mean that a few sizes fit most men and dramatically reduce the store's investment in inventory. Further, the simple construction and reduced cloth requirements mean considerably lower prices. The combination has relegated the pajama to a specialty item.
All of this presupposes that a man wears some form of sleepwear in the first place, of course. The young often dispense with it altogether, at least until they have children running around and grandparents visiting. But though we may be warm enough in our dwellings these days to dispense with clothing, nudity leaves most people feeling just a little vulnerable. Even a towel makes us considerably more confident in the face of unexpected visitors (Mr. Bond's handgun is probably a step too far).
For myself, I still prefer pajamas. I can waken, take my coffee and sit down to read without dressing, and without catching cold in the frigid temperatures my wife insists upon at home. I still get them in linen from Joe Hemrajani
Off the movie set, towels simply do not fit well enough.