While brooding over the history of knitting at the gym today (hey, I've gotta think of something to distract me from my knees), I came up with another reason why the spread of knitting might have been rather slow. One that has little or nothing to do with culture and trade routes, per se.
Mind you, I'm talking about the Middle Ages, here, around 1100 CE.
I wonder, if perhaps the spread of knitting was slowed by a lack of decent knitting needles. Sounds crazy at first, but think about it. Back then they were knitting at really fine gauges (seven stitches to the inch, or less, average) and needles out of wood, that small, kind of suck. Not to mention, even making needles out of wood, by hand, can be a tricky proposition. (Look into arrow making, if you don't believe me.) Not impossible, of course, not by any stretch, but certainly a pain in the keester. And of course, the harder the wood, the more useful the needle is, and the more difficult it is to make in the first place.
Metal needles would be even harder to get. Iron and steel were both very expensive. Bronze and copper would be too soft to be useful (bronze, if pure, would work, but likely be expensive, too). And aluminum wouldn't be made in any useful, affordable way, until at least the 1880s (useless but interesting fact of the day: in the 1850s, aluminum was worth more than gold, because no one knew how to extract it from bauxite). Aluminum knitting needles didn't exist until after WW1, to my knowledge.
There are, of course, other things to make needles out of, bone and ivory and tortoise shell being the most common. And those are lovely, and certainly did exist. But again, the material isn't strong enough to make really small needles with.
I do know that lace knitting, at a very small gauge, did not exist in any real way until the Victorian era, because that was when wire-making technology made it possible for the masses to buy steel knitting needles cheaply (or make them out of bicycle spokes, or the ribs of umbrellas). Until then, only a few people could afford the needles to knit the really fine lace; I'm sure it existed, but it wasn't widespread until then.
So. More food for thought.