I can't be bothered to give ten reasons, so I'll give as many as I can think of.
1. Price. The price difference between the 360 and the PS3 is massive, and we've been through the longest recession for 80 years (where live, at least), so this has been an even bigger factor this generation than it has been in other generations. The price won't deter the hardest of hardcore gamers, nor will it deter rich kids, but it will deter the average Joe and it will certainly deter parents deciding what console to buy for their kids, which is probably why the 360 has been particularly successful at Christmas (it wasn't this year, but that's because parents could finally afford to shell out $300 for a PS3).
2. Poaching Sony exclusives. The minute I heard that GTA4, the successor to the PS2's biggest franchise, was going to be multi-plat, I knew it was bad news for Sony. It got even worse for them when FF13 was announced to be multi-plat as well. Exclusives are one of the most important factors for many in determining what console they'll buy, so the PS3 looked like a much poorer choice when it's biggest name exclusives turned out to be not so exclusive at all. Interestingly, exclusives have been the most important selling point in the 360's weakest region. A huge proportion of 360 sales in Japan have come in weeks when some timed 360 exclusive JRPG came out.
3. The size of the American market compared to the Japanese market. Sony's homeland and it's strongest area (when PS3 sales aree compared to 360 sales) is Japan. Microsoft's homeland and its strongest region by far is the US. Four times as many home consoles have been sold in the US as in Japan and almost six times as many HD consoles. Microsoft's strongest market, and the market that is entirely responsible for the 360 outselling the PS3, is many times bigger than Sony's home market. If the two markets were the same size but kept the same proportion of sales, the PS3 would easily be leading worldwide and would be pulling away every day.
4. Xbox LIVE. It just seems to have taken off in a way that PSN hasn't, despite the former being a pay service and the latter being free. Since this is the generation in which online console gaming has really taken off, this might be a significant factor in the 360's success.
5. Marketing. Nearly everywhere you look, the 360 is being marketed better and more prominantly than the PS3, and the situation was even worse a year ago. There are far more 360 adverts than PS3 ones on British TV (or there were the last time I checked), and in nearly every online shop the 360 is advertised ahead of the PS3. I imagine it's the same in physical shops as well.