Considering that during Vietnam war those field crews managed to restore about half of all damaged (mainly by fragments) missiles into working conditions, I think you seriously underestimate both them and quality of components, honestly.
The S-75 missiles were from the beginning designed to accomodate a mobile system, working in field conditions. They were not intended to be stored in bunkers till ordered to launcher. So it's more likely reliability was traded for required ruggedness - the missile must work in far suboptimal conditions, even not as reliable as in perfect ones.