Why does Itoshiki-sensei always wear a kimono? Itoshiki-sensei is always seen wearing a kimono whereever he is, except in episode six when he is in his hometown, wearing a shirt, but after that he change his clothes to a kimono again.
The show uses a lot of traditional Japanese cultural imagery, including the characters' clothes. Wikipedia says the style is supposed to be taken from Showa period (most probably 1926~1989 years, less likely 1312~1317).

It's a comedy and satire show, so naturally, the main protagonist would wear something noticeable as a traditional kimono. But because characters are usually seen in modern environment, things like this keep us from forgetting the older-timey setting.