Because it runs in any browser, even mobile ones and requires basically 0 time for compilation and no plugins as opposed to unity, unreal or other engines.
I just don't quite understand why everyone is getting so excited about individual demos.
One thing that is regular on WebGL demos is that it is always a mixed experience on my devices ranging from Android 4.3 to 5.0.
All are from Samsung and Asus.
Native demos work 100% without hiccups.
Nice idea, execution is still lacking on mobile.
I am not missing the point, as 99% of the WebGL demos I see being thrown around don't run out of the box on mobile devices that most people have, as you imply.