In-universe, they never really explained it, other than mentioning that it was something to do with the events of the "reboot".
Out of universe, it was largely because the actor (Keith Hamilton Cobb) continually complained about them and because they were a hassle to keep applying:
COBB: I was happy to see the blades go. I never liked them. They were a poor prosthetic made of rubber that you can actually see
bending if you look closely at a couple of episodes. The way they
chose to have him lose them was, I think, somewhat clumsily executed,
but I was relieved to have them gone. I would have much preferred to
have had him walk around with the broken remnants protruding from his
forearms for an episode or two, with stories of how he had shattered
them all smashing them repeatedly into the black, obsidian-like face
of that alien whatever that had taken him captive. But instead, they
just sort of turned up missing.