One thing. You broke the brushes. They touch the "slip ring" on the center of the rotor. The slip ring is simply two copper rings, attached to either end of the coil on the part that spins. When they wear, they develop deep grooves. If they weren't work, I believe the brushes would have slid straight off. So they likely got caught in the grooves and broke. The slip ring isn't terribly hard to replace. There are videos on here showing it. If you didn't change the slip ring, it could cut all the way through the slip ring, and (hopefully) stop the alternator from working. The other thing could be that it makes a short at the brushes, since they'd both be touching the same piece of metal. The slip ring is a normal wearing part, and would probably be replaced when rebuilding. If you don't remember what condition they're in, it's probably worth taking a look. It's better to have it perfect, so you can get 100k miles out of it, rather than it breaking in 10k miles or less.
Yeah, I checked the slip ring and it wasn't worn or anything so I felt ok leaving it as-is and installing the new voltage regulator with the new brushes. This was one of those dumb $50 mistakes that I hope I only ever make once.
