Examples:

Shtop instead of stop
Shtraight instead of straight

I know that many Yiddish words begin with the sh sound (schmuck, schlep, schpeel etc.) but how many Americans know Yiddish or the German from which it derives?

Most changes in pronunciation come from it being easier to say the new form than the old. I could of gone is easier to say than I could have gone. The SH sound is easier to create with the tongue than the ST (try it!). So that must be it.

How does this relate to things economic? Simple, it’s more economical for the tongue to produce SH than ST!