The DC heroes are all pretty expensive propositions. Green Lantern is actually not a sure-fire thing...and I would suggest that it is likely that it's the riskiest choice out of the top tier DC heroes.
So, if this massive risk pays off, then they will start green-lighting more movies. It makes no financial sense to flood the market with risky, expensive movies when you have barely tested the waters.
My guess is that WB is cautiously moving ahead on scripts and initial details...
If Green Lantern tanks...expect a scaled back announcement slate at next year's Comic Con revolving around Batman and Superman. If Green Lantern is a hit (even moderately) expect things like Flash and Wonder Woman to have some huge announcements.
With the studio currently bankrolling Green Lantern, Batman and Superman films (in addition to all of their other franchises), you really can't expect much more from them without it being a release slate nightmare.
And the cautious approach is what should be applauded by the DC fan base. WB could have easily went ahead with a third Batman film without Chris Nolan, but to their credit they waited until Nolan was ready. How often does that happen? And look at the god awful JL Mortal film that almost happened, obviously other factors came into the equation but someone at WB at some point obviously saw the light and realized they were doing things backwards. If Marvel feel like throwing caution to the wind by releasing all these films within a 4 year period that's their decision, but it shouldn't come as a surprise to fan base and the studio heads if things don't go to plan, I actually think what Marvel is doing is quite reckless.