Even in her own country she is equally loved and hated. Her policies have (as some called it) blighted the UK with abject poverty and serious social problems due to high unemployment and the downfall of industrial Britain.
So for a world leader from way back when, why should a more senior group of dignitaries go? Reagan was president of the US during the cold war and leader of his country. While the PM is a high function in the UK, the Queen is officially the leader of the UK, not the PM.
Now if Queen Elizabeth the second dies more senior people from the US will go because she is the leader of her country, for a PM from 23 years ago, the people that are going are more than adequate.
And Obama has more pressing issues than going to memorial services in the UK (making the cost of the funeral go up even more).