1. The site audience tends to be a little older and not very tech-savvy, so Bitcoin isn't something that many are interested in comprehending.
2. Since the price was fixed at 2BTC a few weeks ago when 1BTC~=35USD, and now 1BTC~=100USD, people will see this is a raw deal. (The USD price for a membership is $65.)
It's #2 that's the hard part. I'm not interested in accepting BTC just so I can convert it to USD, but since everyone's so focused on the BTC->USD exchange rate as the measure of value for BTC, and since the rate is so insanely volatile, it's impossible for a merchant to fix a "fair" price without auto-adjusting the BTC price to the USD exchange rate.