of the top of my head I'll say that the biggest deterrent for developing new technology is that nobody wants it. You have to take several things into account. There were no school system for the poor, which is 99% of the population. The rich/noble, had it made allready - they got a free for all buffet of food, women and wine.. So they don't really need anything better. So it will take a long time before things get develloped. Then when you add magic to the equation you get yet another factor - magic can solve a lot of things as you yourself point out. The poor can't afford it though, so they are no better of with or without magic. (unless there is free healing). The religion thing is also relevant - I can hear the shouting of heretic and see the burning torches as the first person steps forward and poses some theory about ... anything *lol*
But mostly - I think its going slow because of lack of interest, people just don't believe they are missing out.
Modern quote by leader of american patent agency (begin of last century) - Everything that can be invented - has been invented!
And if you analyse technological progress - you'll see the curve is exponetional - so the more we devellop the faster we devellop the next stuff.
However, some fantasy involves inventions, steam engines, gun powder and you name it... so some fantasy settings could have some "modern" elements. They could be new or a reliq from an earlier civilization. I believe that the D&D setting Eberon has steam power involved, never played it though. And also Gnomes are often concidered very inventive - so you could base inventions comming from their society and spreading... indoor plumming for instance...