Singapore airlines has it's main hub in Singapore.
Singapore is far away from many cities in Europe, but a 747-400 can just make it to London.
For it's flights to the American continent, Singapore Airlines has to
make a stop with 747-400 and 777 aircraft. Only their Airbus A340-500
flies non-stop to the USA. All the other flights must make a stop
because they don't have the range to reach the American continent