A large number of the holders (especially the foreign holders) bought the Greek bonds precisely because of the higher risk, which resulted in them having higher interest rates than, say, American or German bonds. They made a bet and lost.
Greek bond chart: http://www.bloomberg.com/quote/GGGB1YR:IND/chart German bond chart: http://www.bloomberg.com/quote/GDBR1:IND/chart
Go back out 3 years, and you'll see that as late as December 2009, the yield spread was minor, and that it didn't start to really diverge until July of this year.