Fallout Tactics was so poorly received that Black Isle said it wasn't canon by any stretch of the imagination.
The Brotherhood of Steel was a very, very small organization. It started when a small military unit(a company? I don't remember its initial size), lead by a Captain, and their families went on an exodus to the Mariposa military base. When command of the unit passed on to the Captain's son, it officially formed the Brotherhood of Steel.
The Brotherhood is made up of three branches. The Scribes catalog old knowledge and expand upon it, they're the professors. The Knights are the engineers of the Brotherhood, they manufacture weapons and ammo, which is what they used to trade with the Hub merchants and why firearms are so widespread while pre-war firearms are so rare and expensive. The third branch are the Paladins, the people who wear the Power Armor and wield miniguns. They're the smallest branch and only serve to protect the Brotherhood. The Brotherhood is not meant to be police and they are not meant to keep or restore order. Their only purpose of the preservation of technology, that's it.
The second argument is against the Brotherhood's inability to fight offensively. The leadership is so weak that it leaves the Brotherhood crippled as a fighting force. In both Fallout games, the Brotherhood, the people with the best weapons and armor in the known world, are completely unable to fight both threats, requiring the help of an outsider. At the end of the first Fallout, it is said that the Brotherhood fades into the background of northern California, openly spreading its technology to all and becoming a research & development house in the newly formed New California Republic. They do help drive the mutants out, but they do so in small, limited combat engagements while making a strong effort to minimize casualties on both sides.
The third argument against this is the Brotherhood schism. In between the two Fallout games, there was a schism between factions that caused an exodus among many Brotherhood members. The Eastern Brotherhood leaves Mariposa and heads east to Nevada, where it remains stagnant. The power of the Brotherhood is on a continual decline through all of this time, and the schism leaves the Brotherhood so weak that destruction by the Enclave was a very real possibility if the descendant of the Vault Dweller didn't come around. Fallout 3 takes place 35 years after Fallout 2, is it expected that, during this time, the Brotherhood went from a constitutionally weak research and development faction to the Army of the Wastes, spreading across 3/4 of the country with the sole purpose of fighting the evil mutant hordes? Come on.