i think it's because the wish/desire/want or condition are usually expressed by some past tense. Both in Rus.and in Engl.

I wish they taught us ..... I wish they were teaching us... Я бы хотел, чтобы нас учили (чтобы у нас были)... etc. plus, If I were not busy, If I had time (Если бы у меня было время) - condition expressed by the past tense.