Finding Chartreuse is harder than it used to be. The monks decided to reduce production.

No, they didn't reduce production, they just stopped scaling it with the rising demand.

Chartreuse got very popular in the last few years, and the monks started to produce more, but now they are at a point where they feel they a) earn enough money for the monastery and b) they claim scaling further hurts the sustainability.

So instead of producing more, they allocate the existing stock mostly to the hospitality business instead of the private consumer.