I'm guessing the only reason they're holding off that long is because Affleck's schedule is already too full to start any earlier.
If rumors are to be believed, Affleck is currently shooting Batman vs. Superman
and Justice League
back-to-back. At some point, Affleck is suppose to do Live By Night
which is slated for October 2016.
Affleck will probably direct Live By Night
sometime next year or early 2016 after he finishes his Batman commitments.
According to the report, WB would develop a script for a solo Batman movie so Affleck could do it after Live By Night
, so probably sometime in 2017 or 2018 for a summer 2019 release. I'm assuming in-between all of this Affleck will want a break.
So if you factor in all of those components 2019/2020 makes a lot of sense.