Far more important than the already stated reasons is marketing. Game software development is cheap - no more than half of a game's budget gets into development, the rest goes to marketing. From the half of the budget that's left after marketing is subtracted, most of the remainder goes into gamedesign, art*, and brand. If you're spending a huge chunk of your budget on software development, you either screwed up, or your game is intended to be a demo for your engine, which you want to re-sell for more money than you want to earn with the game.
There are people arguing that multiplatform support is ridiculously expensive, but these are the same kind of people that used to say every dialog needs to be modal and non resizable - they don't know what they're talking about. Games are unlike any other software in that most titles are pretty much developed from scratch. This allows games to easily and cheaply be developed for multiple platforms from the start, with small to almost negligible additional development costs (compared to the main software development cost, art cost, and marketing). Multiplatform development is only somewhat expensive if you write for one platform first, and then later decide to port to / rewrite for another platform.
What's expensive is the marketing of the game, and having a brand. To make an obvious example: Any decent Civilization clone will sell 5 times as much if it's launched under the "Civilization" brand. The cost and the money is in the brand, not in the software.
So how is that related to exclusives? Exclusives get significant exposure due to being exclusives. The reason is threefold:
- Exclusive = money. When you make an exclusive deal you get more money per sale.
- Exclusive = free advertising. XBox events will highlight XBox exclusives, PS events will highlight PS exclusives. There will also be console-branded ads for your game where the cost for the ad can be split under some circumstances.
- Smaller target group for advertising. No need to spend money on a PS gamer site if you have an XBox exclusive.
How much more money you get, in what form that money is (higher percentage, up front loan, or just cash), what the terms of the ads are, and if the smaller target group for advertising males any sense at all, are all things that vary game by game.
*Art = Music, sound effects, models, level design, textures, shaders, etc.