um, for lots of reasons.
1. I already have 2 consoles, and there's no room for a third console in the tv cabinet.
2. 3 consoles would be overkill - at least in my opinion.
3. Relatedly, the 360 adds very little to the ps3.
4. Not willing to shell out extra money for a wi fi adaptor + xbl gold to play online - not that I would play online on my 360 anyway which negates one of the key selling points of the console.
5. I've spent enough $$$ already...I don't want to spend more, and I'd also prefer to spend my money on other things.
6. I just don't want to get one.

I should say, here's some of the conditions in which I would buy a 360:
1. an exclusive that interests me enough to go out and buy a 360. There are only 2 I can think of: mass effect 2 and splinter cell conviction - neither are out and both will likely be on pc.
2. Lack of a particular genre on ps3 that is covered by the 360. For example, JRPG's. Though I'd have to be really desperate to go out and buy a 360 to play a JRPG and it's probably not feasible anyway b/c I live in Australia and we tend to get those 360 JRPG's VERY LATE (i.e. we still don't have ToV here) which negates buying a 360 in the first place.
3. Lastly, some random desire to play fable 2 or banjo kazooie nuts and bolts or gears of war 2 - I already don't have a strong desire to play any of those games but hey, I may suddenly wake up and feel my life is incomplete untill I play banjo + kazooie. I doubt it but you never know...

Basically because of a few personal reasons related to Microsoft.

And besides Im pretty satisfied with it. Also Im not a PS3 only owner ;).

Of couse there is something better:

Being owner of high end gaming pc and ps3.

Beetween those two i get 99% of interesting games I could play on x360.

For remaining 1% - LO alone is not worth 200$, ToV is not realesed in EU, SO4 is not published yet so i don't know if it's worth playing.

i jumped in near 360 launch, good console but i game more on ps3.

Because i have a PS3 & Wii, both of which are reliable. I just don't trust the 360 to be reliable and i don't see why i should pay ahead of time for MS's R&D on the 360. I do feel i am missing out on a couple of the best game this gen though with Mass effect and Crack Down.

There are only 2 exclusive games I´m interested in...Ninja Gaiden 2 and Star Ocean 4, the rest I want are all multiplatform titles...so, 2 games aren´t enough for me to buy a system...besides, I can always borrow my brother´s 360 to play these 2.

I have a gaming computer, and a ps3. My ps3 (60 gigs) died around october and I haven't had the money to repair it. I borrow my friends 360 and another friends wii. I can enjoy all consoles, except the one I bought. kinda ironic but meh no reason to buy a 360 or wii when I can use em for free.

Noise, reliability, too few interesting exclusives, I don't want to pay for online play.

I would get one for 69.99euro

because I don't even have enough time for all the PS3 games I want to play