Because Blizzard are making it pretty clear that they're not looking at attracting endemic e-sports organisations, they're interested in the bigger money from traditional sports orgs. They've not said there'll be any incentives for existing orgs in the OWL, in fact they seem to be heading towards the opposite, where teams pick and choose from free agents. Therefore it's better for orgs to drop teams, see how it goes, then if it's looking up again pick up players from the ever-growing free agent pool.
The doomsayers will tell you overwatch e-sports is dying. I think (hope) in reality the scene is going to change dramatically, and the scene as we know it now might die, but to be replaced with something bigger.