It's about supply and demand and fuel efficency. A small airport between Albany and NYC won't have enough people to fill a plane capable of flying to LAX efficiently from a fuel perspective, so they'll take a flight to NYC first then the LAX on a bigger plane that can spend less per person per mile to get there. Larger planes tend to be more fuel efficient (assuming all seats filled) than smaller ones which is why hub and spoke model makes a lot of sense. The 787 is actually abnormally fuel efficient for its size and it's opening up many direct routes that were previously not economical since it's a smaller plane that costs the same per mile per person as much larger planes.