There may also be a desire not to expose passengers to low oxygen levels for too long. While the passenger oxygen system will supply sufficient oxygen for survival in the short term, people can develop issues if suddenly raised to, and kept at, a high altitude.
For example if you drive up Mauna Kea in Hawaii, ascending from some 2500ft at the saddle between it and Mauna Loa to the peak at 14000 feet, it is not an altitude that you definitely need oxygen for. However if you don't rest at the half-way station you are quite likely to suffer altitude sickness. Some people suffer altitude sickness at the peak anyway, and certainly their movement, cognition, etc, is affected. This would also be affecting the cabin crew who don't have the long-duration oxygen supply of those in seats 0A and 0K.
Passengers with pre-existing circulatory or breathing problems (including a lifetime of puffing on the evil weed as in many parts of the world) may well not tolerate FL140 indefinitely, or even for a fairly short time. This time at F140 may be a compromise between allowing routing over higher terrain with the 30min@FL140 to escape to lower terrain and a higher chance of long term passenger health effects.