The security and technical challenges aren't as high as most people seem to think and are pretty familiar to people who work on the technical side of trading in the financial world. I would imagine though that most of the people with the knowhow are already in jobs lucrative enough that theres little incentive to write a BTC exchange.

Or at least that's how my calculations go - while I've got the technical and financial background to make me tempted to write a derivatives platform for BTC, I already make a decent salary and my perceived risk vs profit calculation doesn't work out to a number that would make it worth my while. Unless I get bored enough to do it anyway!