First of all, I doubt we'll ever get Before Crisis outside of Japan. Even if they ported it to a handheld system, the online features wouldn't work out very well.

That leaves Advent Children and Dirge of Cerberus. There's time elapsed between the two, it makes sense to me to release the first and then the second. Give people time to get their hands on Advent Children and then shove Dirge of Cerberus down their throats.