Once they closed schools early for the year it made sense to close Disney. Does it really make sense for those families on sudden holiday to go running to a theme park? I'm expecting to hear similar from Universal.
As for the re-opening date, well, it gives everyone a bit of time to see what happens with coronavirus. If the spread seems to have stopped, they reopen just in time for Japanese families' planned vacations.
Of course, if not, I'd guess the shutdown will be extended.
But these are all my unofficial guesses. I have no inside knowledge or epidemic expertise.