This is issue not only on Westeros. Most fantasy worlds are similarly stagnant, usually in medieval-level of technology and society. Why they stopped, when our world didn’t? What is different? Well, the answer is: magic. When the arcane knowledge gets to a certain level, it begins to act as a brake against further development.
Technological stagnation is simple: It is easier and cheaper to solve some things with magic, than with tech, so it is magic what is advanced at first. And then, when magic reach some limitations, you don’t have the non-magical knowledge to use tech — you would have to replicate a lot of what magic do before you could have a better machinery. Also, as most of the smart people study magics, no one remains to study physics.
The cause of social/language stagnation is somewhat less apparent, but I have three hypothesis for that too. As all explain generic stagnancy, they can explain also the technological stagnation:
- Small group (or even a single person) with enough power to control the world, to keep it where they need it. So, more (grey eminences) or less (dictator) subtle pulling strings to enact proper edicts, forced migration of enough of common people to smooth some language differences, removal of persons who would try to change the status quo… And of course, this situation allows also for control of technological level. An example of such person keeping the world stagnant is Lord Ruler in the Mistborn series, who actively suppress technological advancements and it is almost impossible to kill him due to his magical powers.
- The magic itself. As we usually know very little about the magic from scientific point of view (Is it another force, next to the strong, weak, gravity and electromagnetism? Is it some quantum effect?), we don’t know how it can affect the people. Perhaps being in a strong magical field has some effect over the way people think and act.
- Or, related to 2), if magic is everywhere, even here, then the people are not homo sapiens, but some other race, either related to us, or just similar, and in such case, their mind may have some subtle differences whose outcome is slower development or even stagnation. The people are simply less inventive than we and more easily satisfied with given situation.
Of course, likely there can be some other explanation, but “the author just made it up and didn’t think it all through” lacks something important… :-)