I think one of the biggest is because so much of the inspirational works that came before were set in similar times and thus a lot of fantasy copy-cats it. However I'd also argue that our concept of "Medieval" is quite broad as a technological period. It's basically anything pre industrial and gunpowder as a wide spread use; but after any period of widespread nomadic lifestyle. That said many Epic fantasy sagas will often include nomadic peoples as part of the story and setting; they are just often not the faction driving things forward.
There's also enough general population understanding about that rough time period that you can build a story without having to detail every detail. You can say "castle" and most people have a roughly similar concept in mind when you say it - cold stone walls, grey, typically thick walls, keep etc... You can then build upon that framework.
It's much like how a large portion of fantasy is heavily based on Western Europeans. Indeed whilst many fantasy stories include many other cultures, they are often alongside or to the side of the standard western "knights and swords and kings and earls etc..." . Again I'd argue that its a bias built into the system partly because of what has come before. However its also likely a bias (esp when you talk of English writing) based on what gets published and to what market and by whom its written. So at some level publishers are making choices which influences what gets published and what writers aim to produce in order to get published - both of which then influences a new generation of writers and readers. A self perpetuating system that every so often gets a wildcard slip through the net that promotes a new style and direction.
Of course the concept of medeival is a bit like our concept of Roman - that of a single dotted point in time where not much changed. When in reality both were long spans of time (Roman was an exceptionally long period) through which there were vast changes. Rising and falling; periods of growth, stagnation, peace and war. That's before you even consider when empires are growing at the interior might be very peaceful, but the boarders or lands afar are a war torn mess. So much of this "common understanding" is built on misconceptions, simplifications and half truths.
And that's just in writing. Lets not forget that films, comics, computergames, other fantasy games (DnD) and TV shows all often aim toward the faux medieval. Dungeons and Dragons must inspire many today, indeed some books are born of DnD games such as Malazan Book of the Fallen. So there's a huge support structure of other lines of interest also having an influence on writers.
That said I am often surprised how die-hard the market often seems to be. Eras like Steam Punk where you'd move into the faux Victorian steam-driven era, whilst a wealth of creativity, often appear to be near dead when it comes to the mass market. Likewise Renaissance, Tribal epics, etc... many areas of fantasy are ripe for conquest in terms of a big market push. I'm sure it will happen one day too. That fantasy fans (perhaps even ourselves) in the future will say. "Gosh why is everything so Steam Punk?" and then some of us or other oldies will recall the "Tolkien and Arthurian Era" when fantasy was dominated by the faux medieval.
