To answer that question requires a brief history lesson. I'll try not to make it too painful...
In the beginning, Verant created the Everquest, and it was good. The citizens of Norrath (the main planet of everquest) were happy and gleeful with much glee, and they set about slaughtering the rats and bats and moss snakes and a_fire_beatle_01's and decaying skelitons and orcs and gnolls of the world. These were happy times in the land, and there was much feasting upon bat wing crunchies and milk (though nary a cow was to be found) and people were happy, except for when they were smote upon thusly by various large objects, including hill giants...
soon however, a note of discontent entered the world, as hardy and strong adventurers banded together into guilds and set out in search of the fabled "Phat Lewts", or items of such spiffyness they would make small children weap with envy.
A great battle was fought, one for the ages, of which scholors would speek of in hushed tones for eaeons to come. It was really cool, but that's not the point so we'll ignore it and skip to the end.
"... And Lo' wheneth the dut cleared, there was upon the slain corpse of an_uber_dragon_01 a super spiffy item, that could be worn by pretty much all races and classes in Everquest at the time."
The adventurers looked at the item, and drooled. they then looked at each other, and realized that though they wee many, there was only one item. Who would get the item of spiffyness?
A great debate followed. Things werethings said on all sides, things that would haunt the classes of norrath for decades to come, and create class divisions amongst some that have not healed to this day.
On of the most vocal of these were the monks.
Their combat and pulling abilities at the time were far better than most of the other melee classes, so they soon developed the logic that since they were clearly the best, anything that could help them in combat was a "m0nk Item". A monks combat proficciency depends on the amount of weight he is carrying, the less the better, so weight reducing items, items with high ac, etc, became highly sought after m.o. Several wars resulted, and after the slaughter was over, it was decreed far and wide that speaing of the dreaded m0nk items would be forbiddon!
That, or illia at some point just got tired of peopal whineing about it and added it to the filter...