It would have been a fairly easy call for anyone with meteorological knowledge that the rain would end relatively shortly after scheduled first pitch. That said, sometimes other things besides weather factor into the decision.
If they start at 6:50 it's likely that it'll be delayed again in the first third of the game or so. However it's also likely that rain will dissipate after 8-9 pm EDT, so if they want to get it in they should be able to.