Because DC legal screwed up. DC Comics sued DC Shoes b/c the DC Shoes logo was too similar to the new DC Comics spin logo. Turns out the outcome of the case was that DC hadn't done their trademark registration properly and they actually wound up being required to pay DC Shoes an ongoing license fee.
Understandably, DC wasn't too keen on the new status quo.
Some have suggested that DC could end up in a similar snafu all over again b/c of the Discovery History channel in Europe... it may depend on whether the usages in DC's trademark application overlaps with usages in the Discovery History trademark application.