As we know, English [ʃ] can be spelt not only sh but also in a number of other ways, as seen in the examples ocean, machine, precious, sugar, conscience, compulsion, pressure, mission, creation. However, sh is clearly felt as the basic way to spell this sound in English. Why? Why did we choose this particular digraph?
Historically speaking, the basic problem is that classical Latin had no palatoalveolars. In consequence, languages which use the Latin alphabet and which do have these sounds have not inherited any single way of representing them.
Greek had and has no palatoalveolars, either. So the Greek alphabet, too, lacks a letter for the sound [ʃ].
In Cyrillic, on the other hand, there is a letter used for just this purpose: Шш, presumably modelled on the Hebrew letter shin ש. This is also the origin of the Arabic ش.
The Armenian and Georgian alphabets also have special [ʃ] letters, upper- and lower-case: Շշ and Ⴘშ respectively.
Getting back to the Roman alphabet, I do not know why the predominant English way of writing [ʃ] is the digraph sh. French expresses this sound as ch. Words that in standard French now have [ʃ], and are so spelt, are (or were) pronounced in Norman French with [tʃ], and that is supposed to be the reason we use ch in English for the affricate.
For our fricative German writes sch and Polish sz. Does anyone know the historical reasons for these choices?
Hungarian writes it with the simple s, reserving the spelling sz for the sound [s]. Czech, Slovak, Lithuanian, Latvian, Slovene and Croatian all use the háček-bearing š. Romanian and Turkish use a subscript cedilla or comma, ş.
We’d better not go into Swedish too deeply: rs, sj, kj.