It takes time to mature a spec after conception. In retrospect, WebGL1
really wasn't ready to go when it was released. In the gap between
WebGL1 and 2, we not only worked on WebGL2, but also invested a huge
amount in making WebGL1 the impressively performant, stable,
consistent, and portable API it is today.
But it didn't take as long as you say, relatively speaking. GLES2 and
3 was 2007 and 2012 respectively. GLES 3.1 was two years later in
2014, and GLES 3.2 (which largely just brings AEP into core) was 2015.
You can roughly pretend that our WebGL 1.1 was our body of extensions,
circa 2014. This brings our cadence up to about par.
That said, I expect things to move faster now that we have our feet
underneath us better with regards to WebGL performance and stability,
and WebGL2 is almost gold.