The B737 was never designed with long sectors in mind, that’s very obvious from the flight deck design with the crew shoehorned into a tiny space and the jump seat resembling a folding camp chair which blocks the door when in use.
The A320 has a more spacious cockpit which can accommodate two jump seats and has tables for the pilots. Airbus simply needs to make the pilots seats a bit more comfortable and it’s perfectly suitable for longer flights.