Por que FL398 é a altitude REC MAX no A320?

I just came across the answer in the AFM it states

"THIS IS THE MAXIMUM ALTITUDE AT WHICH IT IS POSSIBLE TO MAINTAIN CABIN PRESSURE ALTITUDE BELOW 8000 FT"

Green Dot Speed (GD) is the aircrafts best lift to drag ratio speed. As altitude increases, the maximum thrust available decreases.

Looking at the graph beneath, if we increase our altitude and get closer to REC MAX then points 3 and GD move closer together because of the smaller thrust margin of the aircraft.

Having these points closer together means we could easily slip into a quick deceleration if speed is reduced. The REC MAX is calculated based on the service ceiling, aerodynamic ceiling and max certified ceiling and is considered as the safe upper limit.

Here is a nice article which covers this information in more detail:

https://www.airbus.com/content/dam/corporate-topics/publications/safety-first/Airbus_Safety_first_magazine_21.pdf