It could also have a lot to do with the differences in provinces. Cyrodiil is the heart of the Empire, and at the time of
Oblivion, the Empire was still doing pretty well for itself. It's more of an organized and "modern" province, and it probably gets the most funding and best technology.
Skyrim is a much harsher environment, and a lot of its population is probably just trying to survive. They don't have the luxuries of rich, rolling hills, moderate weather, or exceptional farmlands. They get mountains, snow, and rocks. Outside of major cities like Solitude, Skyrim's society is a lot more grounded.
So I think province has a lot to do with it. Summerset Isles or High Rock might be more technologically advanced than Valenwood or Black Marsh. Of course, they all have unique conditions to overcome, so they could all very well be advanced, but it different ways that may seem more primitive to other cultures.
Then, you have the 200 years between
Oblivion and
Skyrim. The Oblivion Crisis leaves the Empire without a ruler, so you have a change of dynasties. Then, the problems with the Thalmor, the Imperial City being besieged, all kinds of conflict—Tamriel is pretty war-ravaged and shaken by the time
Skyrim takes place, and the Empire is falling apart. A lot of technological advancement may have ground to a halt, or even slid backwards into a sort of Dark Age.
Now, don't take this the wrong way or anything, I'm not disagreeing or trying to say you're wrong, so take no offense.
I'm just trying to figure out reasons why it may seem that technology has stagnated. If anything, military tech should be getting more advanced. War often promotes technological growth. Some of mankind's greatest achievements began in the military, then were later adapted to civilian use.