Can somebody please explain to me how the president cannot atten

Wildwood - Can somebody please explain to me how the president cannot attend the funeral of Supreme Court Justice Scalia? He managed to go to Walter Cronkite's funeral in 2009 and Nelson Mandela's in 2013. I have yet to read any news source that can cite what was so much more important. Yes, he did "pay respects" Feb. 19. Was that because it was "on the clock" and not his weekend off?