It is customary and appropriate when a former head of state dies that the U.S. will send, as representatives to the funeral, former high U.S. officials who personally new and worked with the deceased. That was done in the case of the Thatcher funeral. The U.S. sent two former Secretaries of State who had both known and worked with Thatcher. Ronald Reagan would have been the ideal representative had he not proceeded her in death and had he not had dementia. Obama came along too late to have known and worked with Thatcher. Of course he would have had the option of attending her funeral, but generally speaking Presidents opt out of playing a strictly ceremonial role when foreign dignitaries dies, just as you or I would generally not attend a funeral of someone we did not personally know. It certainly was not a snub for Obama to not have attended Thatchers funeral. On the other hand, I wouldn't be surprised if several former U.S. presidents attend the funeral service for Nelson Mandela. If they do, it will be because they want to pay their respects, not because they feel obligated to go. Thatcher was certainly a very important figure in British politics, but the esteem with which she was held world wide would hardly be comparable to the esteem with which Mandela was held.