Celebrim
Legend
For instance, medieval people had the technological knowledge to make crude steam powered devices (I seem to recall even way back at the Temple of Zeus, the doors were steam-powered), but they lacked the critical mass of resources and population to make it worthwhile to develop a railroad. It was simply infeasible, because they didn't have enough people to mine the necessary quantities of iron and coal, nor to construct standardized train cars.
There are other possibilities worth considering as well. The Helenistic Greeks and later thier Roman heirs had the critical mass of resources and population to begin an industrial revolution, but ultimately proved to be politically and socially unable to do so. The Roman economy was so tied to slave labor that the Romans saw industrial power like waterwheels and windmills as both unnecessary and socially destablizing and so took little interest in them except as novelties. So the Roman economy got geared toward producing higher and higher population densities rather than more and more productive citizens. As a result, when there was a shock to the economy there was a huge population crash, which in turn sent the whole society into a death spiral that took 500 years or more to turn around.
We think of the middle ages as a backwards period, but that's really only true of the very early period that no one thinks about when we think of 'medieval'. In fact, by the 11th century the medievals were rapidly becoming tech nerds, and by the 13th century they were in a full blown industrial revolution that was temporarily set back by the black death but which directly lead to the reinnaisance. By necessity, because of the low population and general unproductive land of Northern Europe, they had to invest heavily in machines.
One possibility for why magic =/= technology might not be economics. It might be social. The society simply refuses to go down that path for whatever reason - divine prohibition, lack of vision, complacency, etc.
Another reason that it might not work is that it might be like the trap in 'The God's Themselves'. Maybe you violate conservation of energy in your universe, but if the result of doing so on a large scale is the pollution of the very laws that govern your universe it might be a losing proposition in the long run. In fact, the mysterious social prohibition against the widespread use of magic might actually be grounded in this bit of lost lore. Maybe it was tried once, turned out to be disasterous and now people have forgotten why its disasterous but do remember the social stigma, axiomatic moral law, or whatever.