1) Lambert--Good fanbase and legitimate star quality. At this stage of the game just has to worry about not being too polarizing and over the top, but he's received virtually no complaints from the judges and he has the least to worry about
2) Gokey--Terrible prolonged Steven Tyler scream at the end should really have sent him home, but if anything him staying emphasizes his super strong fanbase. I think he buys into his own hype too much. He's a feel-good story fans must like, but he's been very mediocre throughout and in terms of singing, the most shaky among the three left. But as a chosen one, he's over Kris here
3) Allen--Huge fanbase got him over the hump, and his consistency is awesome--he plays only within a small area and thus doesn't show great vocal versatility, but he's very good at putting his own twist in songs with good lyrical interpretation. He did better than I thought this week. Consistency + fanbase will do that for you even if the judges are trying to throw you under the bus
Allison left sheerly because of her lack of fanbase--she just doesn't connect as well to the audience as the other three, even though IMO she's one of the best singers ever in Idol--in terms of her Idol trajectory sorta reminds of Season 1's Tamyra Gray. She had a very good rock week, but when you just put it in the hands of voters, it's about connection--she just didn't have it. Not to mention that rocker chicks (Glocksen, Overmyer) just don't really do well on this show.
Also--I would like to commend Kris Allen more, because man, when I first started with these rankings I put him at #7. How he went to #3, when the competition gets steeper, is beyond me. Sure several early chosen ones--Grace and Rounds--dropped out quite early, but still...