1. Higher attendance at Epcot compared to DCA
2. Different demographics. DLR has a much higher percentage of frequent guests who don't go on every ride every time. WDW tends to have a greater percentage of one time or infrequent guests to try to hit every ride every time (especially the headliners). Most everyone who walks into the gates at Epcot seeks to ride Soarin' in a given day -- adding other great rides doesn't change that, it just meets that every person will want to ride Soarin', Test Track and whatever awesome Imagination or whatever ride is added.
I don't buy this excuse. If a major ride goes down in any of these parks, the lines for the other attractions just go up. If epcot had more high end draw attractions FP+ would be distributed better since people would have to make a choice.