Tout simplement parce que devoir est l’un des auxiliaires habituels[1] pour exprimer l’obligation en français. En anglais on retrouve must et should, mais en français la nuance d’adoucissement entre les deux est plutôt reflété par l’usage du conditionnel. Le passage du conditionnel présent au passé reflète la même alternance de temps de la forme anglaise.

[1] L’autre est falloir, mais comme il requiert une syntaxe impersonnelle et qu’il n’a pas le même sens au conditionnel, devoir est plus pratique à enseigner en premier.

This is actually a fairly interesting question. The first thing that has to be said is that “should” has two functions in English: either as an auxiliary conveying what more synthetic languages such as French class as “the conditional tense”. Or to convey a value judgment regarding potential future actions.

It is further complicated by the fact that in a correct/formal register “should” should be used instead of “would” for the 1st person singular and plural of what the English calls its “conditional tense”: “If I drank twelve pints I should fall over/vomit” is correct, but most people these days would say “I would fall over”.

The confusion perhaps comes from the fact that French conveys the “value judgment” idea by using the conditional of devoir. In other words, French “says” that “You would have to ring your mum” is the same as “You should ring your mum”.
As an Anglo-Saxon I can’t see how these two phrases can possibly be “identical”, so I suppose the answer is that the conditional in French has uses other than a clearly conditional meaning: bearing in mind that one non-conditional use of the conditional is reported speech with a sense of uncertainty: “d’après les dernières informations les pourparlers auraient commencé…”.

I think the point here is that French can use the conditional verb form to convey a sense of “distance from an assertion”, either due to uncertainty or for reasons of courtesy: it is more courteous to say “you should ring your mum” than “you must ring your mum”.