If they made the plane more comfortable then they might have a winner.
Slightly more space for the same ticket price as the faster planes and they would sell seats.
That slower flight time also means that airlines will lose that plane's availability while it's still in the air instead of being turned around for the next flight. Instead of five flights a day the airline may only be able to squeeze in four flights.
When you're paying $100 ~ $150 million for an airliner, you want to make as many flights a day as possible.