@Justicar
The "s" is the Voiceless alveolar sibilant, but the "sh" is the Voiceless alveolar retracted sibilant.
(source: https://en.wikipedia.org/wiki/Voiceless_alveolar_fricative)
Languages change. The cause may be cultural such as happened with the Norman invasion of Britain under William the Conqueror when English gained many influences from Latin. The cause could be a shift in commonly used phrases that make the change of the tongue from one shape to another more difficult or easy.
@DoubleD i do not care if there is a scientific term for it, it's just horrible on my ears. Listening people, native English speakers, talk about "doctor shtrange", and going to to the gym to get "shtrong" and "shreaming" online....aaaargh
@Justicar For example, it is easier to say "she shells she shells by the she shore" than to say "She sells sea shells by the sea shore."