Apparently the Obamas didn't attend William and Kate's wedding because of the cost of the security for them and all the palaver that goes with it. They were due on a State visit a month later, so there was no snub. But the Royals are supposed to stay politically neutral, so heads of state who are invited to Royal weddings are usually family or from the Commonwealth.
As mostlyharmless said this wedding is not a state wedding, it's a family wedding. In the past Trump has made inappropriate comments about Diana and Kate. Meghan has spoken our against Trump, so I really can't see that he'll be invited. There's been enough fuss over a possible state visit.
Imagine the protests in Windsor if Trump was invited.