Long head tube is probably there to facilitate a decent handlebar height. I've got a 23" Raleigh MTB frame from 1991 where the HT is 182 mm on the actual frame steel, and if you include the upper and lower headset bearing races then it measures to 190mm. Seems normal enough for a large frame to me, just take a look at the HT length on any large sized traditional steel road bike frame, some are even longer!