(I'm mostly playing devil's advocate here, I don't actually think HTML will be suitable for high-end games in the near future. But I think there are decent arguments to be made...)

Then why are they implementing an API that's been unpopular for years and is now being phased out entirely (for AAA games)?

Mobile. They picked OpenGL ES 2.0 for WebGL because it had comprehensively won on mobile. Apple went with ES 2.0 and Android followed. It's taking a long time for the mobile industry to migrate to ES 3 (which would allow WebGL 2) but ES 2 has been a decent stable baseline for a good few years now. That's quite unusual, and very helpful, given how fast-moving everything in tech is.

[Edit to add: ES 2 is based on GL 3, which was the first version to add programmable shaders. That was a huge API change, and an admission that the D3D approach was better. It's barely 10 years old. So any "GL is unpopular" arguments based on the old fixed-function pipeline are a red herring, I think.]

Mobile was and is more important than either desktops or consoles, because the mobile market is huge and growing, while desktops and console are at best stable.

Google came up with a clever technical solution (the ANGLE library) to emulate ES 2.0 on top of Direct3D, so that sidesteps the technical problems of OpenGL on Windows.

Now, for AAA games, desktops and consoles are obviously far more important. I think there are two responses to that:

First, a bet that mobile will gradually catch up and become equally important. There are a lot of factors involved, but on raw technical terms it's not such a bad bet. Mobile hardware iterates very fast, and some mobile CPUs are getting very competitive with desktops (recent iPads and iPhones especially). Sustained performance is an issue, as mobile devices have much stricter thermal limits; but you can put the same mobile SoC in a bigger box, like the TV set-top boxes that Google, Apple, Amazon and others are experimenting with.

Second, there's no reason WebGL 3 couldn't be based on Vulkan. WebGL 2 hasn't even been fully adopted yet, so it would obviously take a number of years to make that happen. Maybe desktops and consoles will have moved on to something newer and better by then, but maybe they won't.

The big question is whether mobile+web is catching up on desktop+console, or if it'll always be a generation or two behind. I think you'd have to be pretty brave to bet against them ever catching up.

I think they're more concerned with the politics of it than the technical requirements of that userset.

I'm sure politics plays into it, but for WebGL specifically, it must have been a pretty easy technical decision. Do you pick the 3D standard used on Windows, or do you pick the lower-end one used by iOS and Android (and can be made to work on Windows)?

You could ask why GL rather than D3D won on mobile in the first place. For that you have to look at Microsoft and ask why Windows Mobile failed (in all its different versions). I don't think you can blame that entirely on politics.

Games are heavy users of driver and card specific extensions for instance. But that'd be at odds with the web's portability commitments.

That's a good point. From a web standards standpoint, it's a very tough conflict to resolve. I think the web people are pushing for common standards. That takes time and it can get very political, but I don't see a better solution. And if they can get it right, portability is a good thing! I don't see why that necessarily means you'll always be behind the curve on performance. A more standardized, portable system can catch up via economies of scale -- it might be easier to learn, have better tooling, a bigger potential market, etc.