In an interview I did with Mike Harding a while ago, he said the remit for his show is 'music from these islands (England, Ireland, Scotland & Wales), and English speaking parts of the world' - this explains why his show contains music from America, but doesn't contain any 'world' music as such. It's impossible to please everyone's music tastes in 60mins each week on national radio. Be grateful we have some folk & traditional music air time on national radio! Sure, some weeks it's more biased towards American music, other weeks towards something else. Take last nights show for example; tracks from Brass Monkey, Emily Slade, live session with Lunasa and Bob Copper - these fit into his remit don't they?! If you're not interested in Mike's show then check out programmes on BBC regional radio stations such as Folkwaves on BBC Derby, Northern Folk on BBC Newcastle, Folk Show on BBC Shropshire. If it's more world music you're after, check out Late Junction or World Routes on Radio 3 - they often contain more folk/trad tracks than Radio 2.
You have to consider what folk music as a genre would gain from more media exposure (aside from bringing the music to a larger audience) - would you want folk & traditional music to compete in the elitist pop world?! As much as I'd like to see a dedicated trad music channel, radio stations (such as the BBC) have to consider if it's economically viable to broadcast such a station. What percentage of their existing listeners would listen to it?