Its opening wasn’t really “excellent.” Not only was it extremely front-loaded, but it came in well below projections. WB projected an OW of $170 million after the Friday numbers (which would’ve still been pretty front-loaded), and it ended up just barely inching past $166 million. Sure, it had a higher opening than any other DC film, but keep in mind that movies like TDK and TDKR didn’t have Thursday night previews or 3D ticket prices, and in the case of the latter, the Aurora shooting likely had a negative impact on the OW.
The bottom line is that after the Friday numbers, the movie dropped off pretty quickly.