Donald Trump thinks he's an important person because he's rich, and he wants other people to treat him like he's important. But he has a problem -- his behavior. He makes fun of those less fortunate, and acts like a boor.
Because of that, he was never accepted into high society in New York. Being rich is not enough to be accepted. One also has to have a sense of decorum, and know how to act in polite society.
He thought he had solved that problem when he got elected. As leader of the United States, perhaps the most powerful man in the world, he was sure that others would have to bow and scrape before him. He was wrong.
When a visit to Great Britain was scheduled right after his election, he made it known that he wanted to ride in the Queen's ornate carriage, and be treated like royalty. But the British didn't want that. In a poll, the citizens overwhelmingly voted to not have him visit. The government then informed him there would be no carriage ride, and in fact, he probably wouldn't even be able to meet the Queen. He canceled the visit.
Then came Prince Harry's wedding. Surely, as leader of the free world, he would be invited. No such luck. Barack and Michelle Obama were invited, but there was no invitation for Trump.
Now he gotten another slight. Senator John McCain has made it plain that he does not want Trump at his funeral. All other politicians of any importance (Barack Obama, George W. Bush, Joe Biden, etc.) will be there -- and Obama and Bush have been invited to give eulogies.
You see, there's a price to pay for being an a-hole. You can treat other people badly and whine about your importance, but it doesn't matter. When you treat people badly, good people don't want to be around you, and they certainly aren't going to invite you to events important to them -- even if you're the president.