One factor to consider is the actor. In some of the cases, the actor/celebrity is actively pursuing SNL, or SNL knows they have a strong and easy relationship with an actor (so it would be easier to call up Steve Martin than it would be to call up a new celebrity).
I don't think SNL turns many celebrities down. I think they're used to getting turned down by celebrities.
For example, when Andy did an impression of Mark Wahlberg, Mark was first indignant in his interviews, saying that SNL has asked him to come on several times before. He also said that he didn't find it funny (or offensive) and he's always trying to get people to do impressions of him.
I think he suddenly realized that it made him popular again, so either he called SNL or SNL called him. Then his interviews were different where he wasn't indignant... he was mad at Andy Samberg. So he changed his answers after he set up the fake fight with Andy on SNL. He went from hating it (and acting indignantly) to loving it as a press opportunity (and being fake-angry at Andy).
Anyway, SNL probably contacts actors (like Wahlberg) and gets denied all the time (or told "I'm too busy" or "maybe later"), so when an actor or celebrity pursues them, then they either say, "We'll schedule you" or "Sorry, what about a cameo instead?" (Which is why they end up with lots of cameos; plus I think they pursue the celebrities for some cameos.)
For example, Paul Rudd and Anne Hathaway both cameo'd in a horrible skit at the end of last season (and both were hosts before that). So they probably were in town and asked or were close enough to the producers that someone knew they were in town and asked.
So let's look at these three: Franco, Lively, and Lautner.
James Franco was already on last season. He probably asked to come back. Then they said, "How about a cameo in 'What Up With That?'" Boom - he did a no-talking cameo.
Then he was probably still talking to them and asking to host again. He did decently enough as a host and in his cameo and is still popular enough where they said yes.
Now, Lautner just had his girlfriend (Taylor Swift) on as a host, and SNL was probably trying to get Robert Pattinson and Kristen Stewart (due to the New Moon popularity). So Lautner was the third on that list. Either they called him, or he was already talking to him (because of Taylor) and they said okay.
Third is Lively. My guess is they called her for the cameo last season in Franco's episode. She enjoyed it and wanted to host.
So, the bottom line is that I don't think they're really being all that strategic and going "only for the younger crowd" or "going for pretty faces who can't act." I think they're being organic and going with the relationships that they manage to establish... same as always.
It's just a coincidence that we've had newer, younger celebs this season (like Meagan Fox, Taylor Swift, January Jones, Joseph Gordon-Levitt, Blake Lively, Taylor Lautner, and James Franco).
First, they aren't all THAT young (January and Franco are 31; JGL is 28).
Second, every young celebrity is justifiable; each one has an illustrious career.
Megan Fox was on TV for years (Ocean Ave, Hope & Faith), in the 2 hit Transformers films (which are actually going up in success), she's headlining films now, and she's going to be in Jonah Hex (I predict a hit), Fathom, and Transformers 3.
Ryan Reynolds was on TV for years (including 3 years of 2 Guys, a Girl, and a Pizza Place), and he's been in Van Wilder, In-Laws, Blade Trinity, Amityville Horror, Waiting, Just Friends, Smokin Aces, Definitely Maybe, Adventureland, Wolverine, The Proposal, and he's coming up in Green Lantern and Deadpool. (He amazes me that he's had 2 Marvel super hero movies and he's got 1 Marvel and 1 DC one coming, and he's hunting for more.)
Taylor Swift was on CSI, is coming up in Valentine's Day, has won 3 Country Music Awards, 6 American Music Awards, 6 CMA Awards, 5 CMT Awards, a VMA, and was nominated for a Grammy. Plus she proved herself by already being a musical guest on SNL and appearing in a sketch.
January Jones is on a show that's been winning Emmies for years. And she's the third person from Mad Men to appear on SNL.
Joseph Gordon-Levitt was on 3rd Rock from the Sun for 5 years, was on TV for years before that, 10 Things I Hate About You, Halloween H2O, Angels in the Outfield, Treasure Planet, GI Joe, a ton of dramas, and he's got Elektra Luxx and more dramas coming up.
Blake Lively is on a show that's been popular for several seasons now, and she's been getting into more movies lately. She was in the Sisters of the Traveling Pants and a new smaller film, The Private Lives of Pippa Lee.
Taylor Lautner was Sharkboy, in Cheaper by the Dozen 2, now two Twilight films, and he's got a third Twilight film (Eclipse) and Valentine's Day coming.
James Franco has been in comedies (Knocked Up, Pineapple Express, Never Been Kissed), TV (James Dean, Freaks and Geeks), action (Spider-Man 1-3, Flyboys), and dramas (Annapolis, Milk, Tristan and Isolde). He's also in Date Night (the upcoming Tina Fey and Steve Carell comedy).
Third, SNL has always featured younger/attractive celebrities as hosts. Sure, we've got a few more than usual this year, but it's nothing new.
So far, for 2009-10, we've had Megan Fox, Ryan Reynolds, Taylor Swift, January Jones, Joseph Gordon-Levitt, and coming up are Blake Lively, Taylor Lautner, and James Franco.
2008-09 included Michael Phelps, James Franco, Anna Faris, Anne Hathaway (all in a row), and then later... Seth Rogen, Zac Efron, and Justin Timberlake.
2007-08 included Seth Rogen, Ellen Page, Amy Adams, Jonah Hill, Ashton Kutcher, and Shia LaBeouf.
2006-07 included Dane Cook, Jaime Pressly, Ludacris, Justin Timberlake, Jake Gyllenhall, Shia LaBeouf, Scarlett Johansson, and Zach Braff.
2005-06 included Jon Heder, Dane Cook, Scarlett Johansson, Natalie Portman, & Lindsay Lohan.
2004-05 included Ben Affleck, Jude Law, Colin Farrell, Topher Grace, Paris Hilton, Ashton Kutcher, Johnny Knoxville, and Lindsay Lohan.
You also start to see the pattern where hosts will want to host two years in a row (like James Franco; because maybe they still had ideas they wanted to do or weren't burnt out by it yet).
Fourth, SNL hasn't exclusively done younger, newer celebrities this season. Drew Barrymore hosted, and she's the female celeb who has appeared on SNL more than any other (they even showed a photo of when she first hosted in 1982). Gerard Butler falls into the category of dramatic actor who they used (and they had a lot of fun with his Scotish background).
- TAE
