Money. Not as easy to develop for as say the Wii or as cost effective but usually games on the 360 sell pretty well. Thus it gets a lot of support. A lot less risk than to develop for the PS3.

 Well it also is much more powerful than the wii...hence it lets the devs do some cool things visually. You don't want to start development for a game like dead rising with an original idea and then realize you can't do half the things becuase the system cant handle it...

I am actually surprised there arent more Wii/360 exclusives. Made for Wii, quickly ported to 360...

sales and the fact that it is easy to develop for helps the decision.

Well I'm sure power has somewhat to do with it, probably more for American devs and less for the Japanese devs, but putting a game somewhere is money based.  Those games probably never would have been made for Wii considering the limitations for the console so then its a choice of PS3, 360, and PC, or just one of them.  Usually they go with all 3 because they can pick the one that would be cheapest to make it on, aka the 360, and then port them over to the others and have a much better chance at making a large profit.  They don't go to the Wii because they don't directly port over costing them more money. 

Although Mr. Stickball seems to have a good answer... I can't help but think that this is it XD

You are forgetting an important cost factor: HD graphics. If you have the graphics and the software on the Xbox 360 it is pretty easy to port this to the PC and even a basic port to the PS-3 is very easy and cheap. So it is rather inexpensive to port between these three plattforms. In the case of the Wii you can't simply reuse the graphic materials.

As it's been stated, the combination of smaller development costs and higher sales potential (both because of userbase difference and spending habits of users) makes the 360 quite more attractive from a finantial point of view.

@ OP - I choose option 3, and a part of a 4th option

The games sell as long as they're of reasonable quality, but the fact that xb360 development is based on Direct X, its easiest to port to from PC development. it takes the hassle off of the programmers this way.

There's always this fight in dev studios:

Team lead - wants to be creative and make something with unique gameplay - pushes for Wii development

Artists - want high end graphics to flex their muscles - push for HD consoles

Programmers - want to get their part done as quickly and accurately as possible - push for 360 development

Audio team - want to use up gigabytes of space for huge soundtracks, need tons of space to store it all - push for PS3 development for Blu Ray storage allotance

Studio manager: make it multiplatform, cater to the lowest common denominator and release for everything under the sun.

Just so they can stick their disc into this.