Actually, I am pretty sure that Garchomp was banned mostly due to Sand Veil. It had other factors going for it, excellent bulk. above average speed, access to Swords Dance, strong STAB attacks in EQ and Outrage, however, it was the abuse of Sand Veil that caused many players to become frustrated with it in the metagame.
Sand Veil pushed Comp over the edge in BW1, due to the fact that it could sub roughly 5 times, turning every 100% accurate move into Stone Edge, and stall for a miss. With the free turn, Chomp could find the time to get a free boost with Swords Dance, or kill your revenge killer with Outrage / Earthquake. The combination of everything Garchomp had going for it, plus the 20% boost to its evasion and what it could do with a free turn, pushed it over the edge and hence, it was banned.