WebGL is only supported in Windows 7 and newer in Chromium.
In short, the way older Windows versions handles drivers cause many bugs and the effort required to maybe make it work is not worth it since these systems are already out of support (XP) or are barely used (Vista has 3% of market share).
Some quotes from the Chromium bug tracker:
"Graphics driver behavior on windows XP is incompatible with the requirements of the 2D canvas API, so it will never be allowed by default. Users who know what they are doing can circumvent this by starting Chrome with the "--ignore-gpu-blacklist" command line option. If you do this in windows XP, your browser tab may become unusable due to a remote desktop session or due to screen lock, screen saver or system hibernation. To recover, you have to reload the page, which implies possibly losing some data."
https://code.google.com/p/chromium/issues/detail?id=346026
"Almost no GPU features were supported anywhere on XP anyway. We supported WebGL and 3D CSS on a subset of XP machines. We have robust software fallbacks for each; 3D CSS should perform quite well with the software compositor. WebGL will unfortunately be slower for the few users who previously had hardware support for it, which is a shame, but XP's at the end of its life and will stop receiving security updates in April. Moving to a world where hardware doesn't need to work on XP will make it much easier to make progress everywhere else (e.g. by upgrading Chrome to use DirectX 11 instead of 9)."
"The new graphics framework (referred to as Aura, see discussion for issue 315199 ) will not support GPU acceleration on XP and Vista. There is no going back. If you are a software developer, you could hack your own Chromium build to make it do what you want, but that would be at your own risk."
https://code.google.com/p/chromium/issues/detail?id=315199