Because they were lead by Hawkeye, and he knew the layout of the Helicarrier.
That's what I assumed, anyhow.
Well I don't want to think about it since I just want to enjoy but my parents kept on bugging me so I did think about it:
1. The Helicarrier turned invisible, but was still spotted
2. Say the the enemy got a tracking device to find the ship, or Hawkeye does, but unlike them, the Helicarrier has no idea that a craft is so near it.
3. The enemy only has small craft, maybe a group of 20-30 men or even less and it was parked outside and its guarded by only a handful.Why wasn't it attacked by the jets parked there? Remember that a jet took off when the nuke order was given.
5. Given the techonology, they could have detected where the enemy entered(just one point) but it looks that even if they did, they don't have something like a SWAT team. If they did, they were too slow since the enemy reached the main control room so easily.
I have no problem with Hawkeye damaging the thrusters with one arrow, he's supposed to be that dangerous. But it just looked like the ship's security is lax. Say the craft of Hawkeye are technically friendly troops according to the radar, the infiltration is still too good to be true for me. I still enjoy the movie though.