For one, the Roman Empire was actually not a very innovative entity. There are a few pieces of advanced technology like concrete which would not be re-discovered until after the renaissance but for the most part the Empire was simply good at achieving the economy of scale through mass deployment of capital to accomplish public works projects (i.e roads, Coliseum) using existing technology.

Second of all the Medieval era had a -higher- rate of technological advancements than the Roman period. The medieval period saw significant advances in agricultural techniques (i.e the two fields/three fields system), equipment (i.e the heavy plow, better shipping) and machinery (i.e the windmill and the mechanical clock). While they "looked" less impressive than the Coliseum those are in fact more important in enhancing productivity and building up the basis for industrialization (i.e the first mechanical looms in the 18th century).

The other big thing is that people have a tendency to emphasis hardware (i.e steam engines, concrete) but it was really "software" (i.e commercial institutions which allows capital to be routed towards industrialization) which was probably the more important factor in industrialization.

Last of all it should noted that the eastern half of the Empire (Byzantium) actually did survive for another 1000 years and did not industrialize. The idea that Rome was a lost technological paradise when compare to the Medieval era doesn't hold water upon closer examination.