Why? You have regressed a feature that your users want. I realize that plugins aren't sexy, but Unity is something that both developers and users want. Entire development studios are based around creating online games in Unity that have active user bases who enjoy those games.
For better or for worse you have removed the ability for people to enjoy using a part of the web. But hey, an idealized vision of software perfection won out over what users wanted, so you got that going for you.
I'm a bit bitter that those in charge of browsers have en-masse decided that being pragmatic is worthless in comparison to creating idealized castles in the sky. This just as much applies to Mozilla as it does Chrome. Mozilla's response to why they won't implement Pepper is just as "pie in the sky, who cares what users want" as Chrome removing NPAPI is.
Plugins are something that users want. Ignoring how Adobe has managed to completely drive Flash's overall quality and performance into the ground, plugins are useful. They may not be useful to Developers who are doing Serious Developer Stuff at Serious Companies, but for a lot of users, my mother, my grandmother, my nieces and nephews, they are very useful.
On the other side, browser developers have content developers as customers. Those content developers prefer alternative toolchains. Flash and Unity have far better tool chains than WebGL does. Debugging problems is far easier when you don't have to go through multiple translation layers. And less layers in the stack means less places for something to go wrong. If you are writing against Flash then Flash may have a problem. If you are writing against 3 different browser's implementation of WebGL and/or ASM.js you now have 3 different sets of bugs and implementation quirks. (Not that Flash works the same across all browsers, but at least it is from one vendor!)
But all of that is thrown away in the name of "web standards".
Users care about what works. As developers we have a responsibility to make users happy, our own sense of happiness is sort of not the primary concern.
