Think about it. Without Defeatist, Archeops is ridiculously overpowered (not broken, but a pain in the neck nonetheless)
With a spread of 75 / 140 / 65 / 112 / 65 / 110, it has a huge potential as a sweeper, physical, mixed or special. Combine that with its speed, makes it a threat to pretty much everything in B/W. For a glass cannon, it has decent HP which cannot be overlooked. The only setback is shoddy defenses but often times without specific counters, it is impossible to take it head on, specially for newer players.
Without Aqua Jet, you can rarely such a fast Pokemon and KO it before it wrecks half your team.
Further, it has a great movepool that takes advantage of its typing and stats. It is easy to see why it can be broken without a check in place.