Probably because they need these planes sooner. I don't think the 747-8 freighter will be ready until 2008/2009.
Its possible that fedex was supposed to be one of the first to be delivered the A380's and had to quickly find a suitable substitute that could start being delivered to them in the same timeframe.
Anyway, this is great news for Boeing and extremely bad news for Airbust.
Let's hope the Dem/Libs/MSM go the way of the A-380.