SDO is the reason. Thomas, Rehnquist, Scalia, and maybe Kennedy would have been the only justices on the right side of the case. They would not have chosen to get involved knowing full well where the other 5 goons stood on the issue.
Not surprisingly, Kennedy has taken a nuanced position on these matters. He is way more open to religious displays than the liberals on the court. But he won't go as far as the conservatives. How he will come out on this one is really a coin flip. If I had to guess I would say he will come out on the right side. I am sure that Roberts and Alito will be rock solid on this issue.
Only four affirmative votes are necessary to take a case.