The FAA requirement for an aft door (door behind the wings) is in place for planes that can hold more than 200 passengers.
Alaska configures it's Boeing 737 Max-9 flights for a maximum of 177 people. Thus they are not required to have the aft door.
Having the extra door makes the plane heavier, consuming more fuel, requires another maintenance and testing issue, and complicates the passenger seating, passenger compliance, and the cabin management of safety awareness for the cabin crew.
It would be interesting to see how other similar USA-based short-haul airlines handle these issues. I looked at the Southwest Airlines website and they have large fleet of slightly older Boeing 737 configurations, none of which are licensed to more than 175 seats.
SouthWest Airlines Fleet:
As of June 30, 2023, Southwest had 803 Boeing 737 aircraft in its fleet:
Type | Number | Seats |
737-700 | 408 | 143 |
737-800 | 207 | 175 |
737-8 | 188 | 175 |
Of note: The 737-8 is the Boeing 737-Max 8 which has been in use since 2017
These other SW Airline footnotes:
- The Company's fleet has an average age of approximately 12 years.
- In 2022, the Company's average aircraft trip length was 728 miles with an average duration of two hours.
- Southwest was the launch customer for the Boeing 737-700 in 1997. Southwest also was a launch customer for the Boeing 737-500 and 737-300 series aircraft. And, in December 2011, Southwest announced it would be the launch customer for the Boeing 737 MAX 8.
- On Oct. 1, 2017, Southwest's first 737 MAX 8 began scheduled service.
(I do not know the difference between the 737-MAX 8 and the 737-MAX 9)