[kj12345]

I wonder if it might also be that Microsoft has 1st-party knowledge of its own OS, and this biases them towards depending on specific new features of the OS in their applications, whereas Google et al have a 3rd-party mentality of just looking at the most obvious and stable APIs.

[bkhl]

I doubt that's the case. When Microsoft needs something to be done, they would do it. If you discard all the technical mind and just think with a typical business mind, calculating the resources and cost to develop the new IE9 on XP, which is 10 years old, you wouldn't really invest your resource, time, and money to develop IE9 on XP. Plus, if IE9 is supported on XP, it will give "hope" and "hint" that Microsoft still cares about Windows XP.

Chrome, unlike IE, jumped into browser war really late. What this means is that they need to support as many platform as possible to gain every possible percent of the usage metrics. In the end, the percentage number of the usage metrics is the key indicator of success.