This is my guess
in the early and mid aughts, the rigid bolide made from heritage leather like box, fell out of favor.
many newcomers to the brand preferred the slouch of a more relaxed bolide, and also the ease of pebbled leather
a structured, smooth heritage leather was considered your mothers or your grandmothers bag.
while heritage leather like box can be refurbished well, the new clients also wanted bags that kept a new look
another thing to consider is that with the increase in prices, there are more clients examining every bag ‘imperfection’ with a magnifying glass, and it may be possible that box or other heritage leathers show those variations more readily?
i have read (I don’t know if this is true) that epsom is pressed from skins that are actually slightly imperfect box.
with the rise nowadays of mini bags, structure is on the rise again, and epsom is simply less expensive to produce and takes color easily. There are plenty of box hides around; my custom bag maker (Duret.com) has shown me vintage box hides and others, but it seems easiest to source epsom.
JMO and of course I would welcome correction on any of these points