Alright, let's look at some of the issues involved in this:
1. One of Obama's first actions in office was having the Winston Churchill bust, an official gift from the Brits, removed from the Oval Office and returned to England, an act that the British almost certainly saw as the insult that it clearly was.
2. Shortly after that, the Obamas' gift exchanges with Britain's then- Prime Minister Gordon Brown, his family, and the Queen were disastrous, deeply embarrassing, and revealed Obama as a narcissistic, self- absorbed moron who didn't know the first thing about Brown himself or good diplomatic relations in general with our nation's strongest ally.
3. Upon visiting Ireland (Which the UK is on good terms with now), Obama gave a public comment in which he openly mocked the Irish accent in a grotesquely offensive, borderline bigoted way. Had Trump or any other Republican done it, the DUmmies would have crucified him for it.
4. In contrast to all this, when President Trump took office, one of HIS first actions was having the Churchill bust brought back to the White House and restored to its former place in the Oval Office, a clear token of his goodwill to the Brits and a sign that he will treat them with much more respect than Obama did.
5. Even if the British royals hold an unfavorable view of Trump, they undoubtedly recognize that America is one of Britain's strongest allies which they should maintain positive relations with, and that diplomatic goodwill is more important than personal feelings about a person.
Keeping all this in mind, you REALLY think they'll invite the Obamas but snub the Trumps?
Also, the Royal Family undoubtedly knows that if Moo- chelle Obama comes to the wedding, they'll need to order at least twice as much food for it as they would have otherwise.