All of the people having a fit over President Trump not attending McCain's funeral barely batted an eye when Obama did not show up for the funeral of General Greene who was the highest ranking officer killed since Vietnam.
They didn't have a fit over Obama not attending the funeral of Nancy Reagan either. They also did not have a fit over Obama not attending the funeral of Margaret Thatcher.
McCain and Trump were rivals with plenty of animosity towards each other. McCain even stated he did not want Trump to attend his funeral. Trump is respecting McCain's dying wish by not attending. I'm sure other presidents besides Obama & Trump have not gone to funerals.
In the case of General Greene & Senator McCain, the family's wishes were honored regarding the decorum for the funeral service that they requested from what I have read in both cases.
