Yes, the Scarlet Brotherhood itself was always a bunch of monks.
What I want to know is, why the descendants of a magocracy empire became a bunch of monks.
The closest to an explanation I'm aware of is the history section of the 2E AD&D
Scarlet Brotherhood accessory book (1999).
A Suel noble called Kevelli Mauk founded the Scarlet Brotherhood as a "racial purification" group a few years before the Rain of Colourless Fire. He had a premonition of disaster and used an artifact to teleport himself and his followers away, where they joined up with various other bands of Suel refugees and founded a colony.
The History section includes this pertinent paragraph:
Although Kevelli died in 5105 SD, his vision lived on. He was succeeded by his most talented student, Reshek Nes. Reshek followed her mentor’s lead and created a strict monk-like regimen for the most talented students, building strength and focus through discipline and denial. Over the next century, the Scarlet Brotherhood’s members developed a method of unarmed combat, recognizing the many situations where an empty hand and an innocent face would be a valuable weapon. Other members trained in the arts of disguise. poison. espionage and lethal strikes. Still others learned stealth, infiltration and thievery. During this time the three-tiered structure of the Brotherhood developed, with martial artists at the top, assassins below them, and thieves comprising the remainder.
So, it sounds like the founder and his protégé favoured the discipline of monks and that class gained a dominant position in the Brotherhood.
Also, the Ancient Suel Empire may have been ruled by wizards, but the original Scarlet Brotherhood was an independent organization in that empire, so might not have had many wizards members. It may have been a piddly little organization that the powerful Suel wizards of the time would not deign to pay attention to, but was in a position to dominate the disorganized Suel survivors because it was lucky enough to survive the Rain of Colourless Fire with its power structure intact.
Also, few of the "magocrats" may have survived the Rain. Presumably their Baklunish enemies would have focused on destroying them first, and many of them could have dies trying to oppose the Rain of Fire.