I am a HUGE folk metal nut....not just the stuff that has been hitting the "trend" so to say like Turisas, Korpiklanni, Ensiferium and such....but stuff like Alkonost, Norgard, Nomans Land, Vokolack, Obtest, Svartsby, and Black Messiah, plus tons of smaller russian bands. It definetly is an aquired taste. I have always had an interest in mythology so it helps a little and I love the mix of metal instruments mixed with instruments of old (mouth harp, mandolin, flutes and such). I think that the sudden rise is that it is something kind of new and different for people so they are going to buy into it.....will these same people go further...doubtful since the releases are so hard to get. The one down side of the folk / viking metal is the nationalist / white power crap that is growing in it. You really have to be careful about what you get. Stuff on the Pagan Front is the most obvious. Basically nazi white power stuff. Avoid at all cost.
I am glad that there are some great bands that are getting the exposure. Like all genres in music...there will be a high point and then it will drift away and there will always be bands playing that genre but the genre hoppers will move on. Remember there was a huge power metal kick in the early 2000's. Blind Guardian, Hammerfall, Gamma Ray, and Angra were all the rage. Then the black metal rise and it was uncool to like power metal. These things all come and go. I have been into folk metal since I started getting Skyclad disc back in 99 or so. There wasnt much to choose from back then. Now it seems that there is a new disc each week.
Sure it isnt for everyone. Like any genre out there. I am glad to be finally able to see some of these bands here in the states.