Why outsource the sequel?

I'm always stunned when a company releases a new game that absolutely kicks ass, and then outsources the sequel to some other studio like it's a requirement of the industry to 'pay it forward'.

BioWare did this too, with Neverwinter Nights 2 AND Knight of the Old Republic 2. Why?
Some may argue that in BioWare's case, they were working on Dragon Age.

In some cases it even makes sense, such as with Quake 4 and Raven. Raven Software has had a long history of working with id Software's materials (engine or ip), and even though the numbering was a little out (it was a sequel to the predecessor's predecessor) it was a great game that represented the original ip to a tee. I feel this is mainly because of the long standing relationship that existed prior.

Regardless, I'm stuck on why a studio would do this. If it's burnout you're afraid of, just don't develop it right away. Convince your publisher that absence makes the heart grow fonder and release an SDK, along with some DLC. Next stop: profit margin.

Two years later, after you're studio releases Barbie's Adventures in Tire Slashing, the artists will have a hankering for that old ip that everybody loved and the game will make itself.

I assume you are talking about Bioshock 2?

2K Marin was created from members of Irrational, and 2K Canberra was once a half of Irrational so to say it was outsourced isn't really accurate in this case.

I can't speak for IG (as I'm just a fan) but my understanding is that Ken and company passed on working on the sequel because they wanted to focus on the as-yet-unnamed new project.

As to the business of gaming as a whole, I imagine it varies from company to company. I'm sure cost plays a part (i.e. maybe Publisher X wants a sequel to Developer Y's hit game but Dev. Y is already working on a new project, so Pub. X assigns the game to Dev. Z)

On a related note, I always laugh when people complain about Bethesda "outsourcing" Fallout New Vegas to Obsidian, when the fact of the matter is that Obsidian is made up of many of the very people who worked on the original Fallout series.