The grant of rank was due to Lord Abercorn's excessive worrying about rank and class distinction. George W. E. Russell's comment on his wikipedia page sums him up:
"This admirable nobleman always went out shooting in his Blue Ribbon, and required his housemaids to wear white kid gloves when they made his bed. Before he married his first cousin, Miss Cecil Hamilton, he induced the Crown to confer on her the titular rank of an Earl's daughter, that he might not marry beneath his position; and, when he discovered that she contemplated eloping, he sent a message begging her to take the family coach, as it ought never to be said that Lady Abercorn left her husband's roof in a hack chaise" (Collections & Recollections (1899), p 77).
Also see the late Leigh Rayment's remarks:
Abercorn was obsessively rank-conscious and went to great lengths to remind the common people of his exalted status. Even before he succeeded to the Earldom, when he was travelling in Europe, he had cards printed which described him as "D'Hamilton, Comte Hereditaire d'Abercorn". His livery was very similar to that of the Royal Family and, when someone remarked upon this similarity, he replied that that the Royal Family had copied it from the Hamiltons.
For his second wife, he married his cousin, Miss Cecil Hamilton, but before doing so he persuaded Pitt the younger, then Prime Minister, to elevate her to the status of an Earl's daughter so that he might not marry beneath himself. In the event, the marriage was not a happy one and, when he discovered that his wife was about to elope with her lover, he was anxious that aristocratic conventions be observed and begged her to take the family carriage to meet her lover "as it ought never to be said that Lady Abercorn left her husband's roof in a hack chaise".
Abercorn's style of living was, even in that prodigal time, extremely lavish. Sir Walter Scott, a friend of the family, once met a procession of five carriages, twenty out-riders and a man on horseback wearing the blue ribbon of the order of the Garter, all on their way to dine at a public house. Since a mere public house could not be relied upon to provide food of the quality to which Abercorn was accustomed, his cook had been sent on ahead to oversee preparations.
He would not accept anything from a servant who had not previously dipped his fingers in a bowl of rose-water, and the housemaids had to wear white kid gloves while making his bed.
Visitors to his home at Bentley Priory, Stanmore, were accorded the run of the house and were free to do whatever they liked, provided they did not speak to their host. Only at meals would Abercorn speak to any guests - at all other times, guests were to ignore him. On one occasion, he was anxious to invite some guests, but when they replied that they couldn't afford the journey, he sent them a cheque. However, when they arrived, Abercorn, having watched their arrival from behind some curtains, decided he did not like what he saw and disappeared from the house until their visit had ended.
For further reading see:-
* The Emperor of the United States of America and Other Magnificent British Eccentrics by Catherine Caufield (Routledge & Kegan Paul, London 1981)
* Brewer's Rogues, Villains Eccentrics by William Donaldson (Cassell, London 2002)
S.S.