Because fees / yield are paid out over 1 year, but only if you hold the BitUSD for 1 year do you get 100% of the yield. Everyone who bails early leaves more yield on the table for those who are holding on.
The end result is that we have a yield pool based on old income when the BitUSD supply was over 600K and interest was being paid. The supply shrank to 150K but the "collected fees" didn't shrink so "collected fees" is now a larger percentage of supply than it was in the past.