Why did the Minotaur cross the Elbereth?
Because he wasn’t afraid!
This behavior was introduced in 3.0.5 for (quoting the link source code):
/* Note: minotaurs must be immune to scare monster * to avoid abuse from creating them and taking * their wands, then polymorphing 60 or so * wands to get wishing... */
By 3.1.0 this note has been deleted (I don’t know why) but the minotaur has still been ignoring Elbereth until 3.4.3.
Source code archaeology sometimes leads to funny insights. As a consequence in the next version of UnNetHack minotaurs will be afraid again, like every other normal monster.