Apparently her condition couldn't be cared with a transplant.
From the Daily Mail:
"Kirsty Howard was born with a back-to-front heart on September 20, 1995.
Her condition, a form of situs ambiguus, led to the misplacement of her internal organs.
The problem could not be cured with an operation - and left Kirsty dependent on a constant oxygen supply for survival.
The youngster was the only person in the UK - and only the second in the world - to be diagnosed with the condition in February 1999
She was initially given only six weeks to live, but defied the odds to reach the age of 20".