Why are protic acids not used for the formation of thioacetals?
The pKa/leaving group ability between alcohols and thiols may have something to do with it.


  Alcohol pKa around 17, thiol around 13.  


As you pointed out the whole reaction is in equilibrium. But in the case of the thioacetal, after the initial addition theres a strong driving force to kick back out the sulfur to reform the carbonyl rather than committing to the thioacetyl formation since the sulfur is a relatively stable leaving group. 

Protic acids are almost always wet, and as such the water would prevent the reaction, whereas Lewis acids are generally either neat, or in ethereal solutions.