Wild speculation: iOS 8 will bring an out-of-process, heavily sandboxed webview, which was the missing piece for enabling webgl for everyone. The greatly increased attack surface (with almost direct gpu access via shaders etc) was too risky to host in in-process web views - explains why webgl was only enabled for iAds (since those would be vetted by a review team)
What really?
Were they seriously even pretending to use that excuse?
Everyone I've ever talked to was 100% certain it was simply because they didn't want anyone by-passing the app store for games and other immersive content they would then have no control over.
I'm not denying that webgl is an attack surface; it totally is; but surely it's a bit rich to say that the webgl implementation in webkit has been switched off for the last 3 years because of security concerns.
If that was the only reason to not having feature parity with other OS's, they'd have some something about it.. oh, you know. 2-3 years ago.