Why do the PS3 and 360 sell differently in different regions?

[Reasonable]

A simplified view for me would be that:

in US/UK MS got the timing right with 360 to adopt gamers who by game preference would have gravitated to PC a generation before, plus they had the existing framework of the Halo and online base from Xbox, plus no competition from Sony at launch. By time PS3 launched, 360 therefore had a strong install base in those countries amongst a key game buying demographic - plus other English speaking countries to a lesser extent

PS3 launched with very high price and due to over-focus on competing with MS, a machine clearly designed more as a full feature entertainment centre than a games console, plus too many new IPs at launch, and a lot apparent focus on FPS and online to try and draw in US, UK, etc. - and in the process made itself the less attractive offer vs the 360 in US/UK plus of course the 360 already had a strong lead as per above with the demographics Sony seemed to be pursuing.

Meanwhile, outside US/UK the dominance of Playstation brand seemed to hold of interest in 360, I think due mainly to previous franchise preferences and the view 360 = Halo and nothing else. Therefore when PS3 launched it became dominant over 360 in those countries fairly easily

Once the battle lines were drawn, changes of momentum are slow, so PS3 has sold essentially the same as 360 in Others with 14 months to spare, whereas in US 360 has kept its momentum and PS3 has seen slow growth

Finally, the Wii launched and looked more like the next expected playstation console than the PS3 did, and it promptly ate up both a fair bit of the former PS2 demographic as well as adding new demographics at the same time.

So in short, 360 I believe has taken little market share from former PS2 demographics but, in a similar fashion to Wii, has actually been steadily adding a new demographic to consoles and building on its own base as started with the Xbox and the success of Halo and Live - the PC / Teen / Young Male demographic. It has of course taken some former PS2 marketshare around titles like GTA, and the focus for this has been US, UK and some other English speaking countries, but it's clear looking at Wii vs 360 vs PS3 vs last gen that it's the Wii that's really ate up a lot of former Sony marketshare, not the 360.

So the Wii has taken a huge chunk of former PS2 demographics, and is the choice for kids, families, etc. plus of course stalwart Nintendo fans - hence its massive sales next to both the PS3 and the 360. As per the PS2 success story this has also seen Wii see consistently better sales in every territory. In addition the Wii has been able to expand into other demographics even beyond the PS2.

The PS3 meanwhile has ended up in an odd place, regaining some PS2 demographic for core titles, but losing a lot to Wii and a little to 360. Fighting with 360 for some of the online/action crowd and gaining some success, plus I think adding some former PC gamers too as well as appealing to those looking for a one stop console/BR Player. It has remained ahead of 360 outside US, etc. but has lost all regions to Wii currently and US/UK to 360 - although UK might even out in the end.

Speaking specifically to Japan, 360 never really had a chance as it had terrible image and library for the region, and by the time the IP, etc. MS invested in arrived the Wii was already on the market and the new local console of choice, with PS3 retaining just enough brand image and exclusives to ensure the 360 had no chance to compete meaningfully with it in the region. Hence the Wii remains ahead, the PS3 has surged with each new injection of core franchises, and the 360 has plodded a tough road to show improvement over Xbox, but still remain clearly a long way behind the other consoles.

Other aspects behind this I'm sure are:

Perceived games library, per country stance on price vs quality, local buying patterns, local position on PC gaming vs console, brand awareness, focus on online vs offline, local preferences for paying for online vs free online, previous game franchise preferences, exclusive game preferences, plus lots more!