Why some people says that Kuroshitsuji/Black Butler doesn't deserve to be called a shounen?

Because their yaoi fangirls who think all anime with boys in it should be a yaoi anime.

Because it's not like the other shounen anime?

Maybe because they judge too quickly o off of the first episodes? I cant say for certain. Ive never watched that anime yet, though Ive seen bits of it. It might be the case I had with Kuroko no Basket before actually watching it. When there are animes that have a lot of 'Sexy MEPs' o have bishie/'hot' anime guys, most people classify it as not shounen and sometimes even shoju anime rather than the shounen they really are.

Its called judging too early.

It's not your typical shounen, it has drama, action, violence, add different elements that aren't in shounen, the male to female ratio is higher(Sebaciel), It has a tone directed to females, but it still has shounen elements, It's no KHR o InuYasha. There have been animes that have shounen título and aren't considered shounen por others (Free!)