Short overly-simplified answer: Universal obtained the rights to use some Marvel properties well before Disney bought Marvel, back when Marvel was having financial issues. Disney must abide by that contract. Universal still retains the rights to use those characters, and Disney cannot use them in their Florida parks as long as Universal decided to keep up their end of the contract. That's why DCA is getting a Marvel land and WDW is not. However Universal can only use the comic book versions of the characters, not the more modern film versions.
As for DC Comics, Universal would have to get those rights away from Six Flags, who use DC characters extensively throughout all of their U.S parks.
-Rob