Anatomy of a third-party platform-exclusive game
The basic premise of a TPPEG is that the developer gets to concentrate on the capabilities of a single platform. The idea being that that this will enable them to make a better game than one where they have to consider the requirements of several sets of hardware.
Making the game for multiple platforms will take extra development effort, which is money that needs to be recouped too, as does marketing the game across several channels.
Also, all things being equal, a better game will sell more than a poor one.
In the case of TPPEGs the pool of prospective gamers willing to stump up their hard earned money for the game has to be large enough on the target platform to make up for the lack of sales due to the limited release.
This all probably makes for a very complicated spreadsheet in some manager's office somewhere. Target demographics, development costs, marketing spend, audience size and more will all be neatly estimated, numbered and crunched until a yes or no answer comes out at the bottom.
Then the platform holders come into play. From help with development and marketing to honest-to-goodness bundles of money, developers can get all kinds of incentives to concentrate on a particular console, each of which will affect the manager's spreadsheet and the estimated profit at the end of the process.
These incentives will come with strings attached – even the early lunch meetings are not gratis in the long run. Usually the idea is that the developer won't port the game to a competing console (porting to the PC seems to be an OK move in everybody's book) although this condition seems to have migrated to a “timed exclusivity period” recently. The only titles that are exclusive in perpetuity any more are ones where the platform holder owns the intellectual property to the game.
One other way for the platform holders to set "their" version of a game apart, even with a simultaneous multi-platform release, is with DownLoadable Content (DLC). Microsoft famously paid a large sum of money
for Rockstar to develop 360-exclusive DLC for Grand Theft Auto IV
for example. I can see this approach gaining more popularity in the future since the publisher won't lose any sales due to a staggered release.