a) Web doesn't need more plugins.
b) Web doesn't need plugins that actually require certain browser to work
c) Web especially doesn't need plugins which have different downloads for different browsers.
d) Microsoft has ditched Silverlight in Web. Silverlight is now basically for Windows Phone only.

e) Moonlight doesn't work. I use Opera and they try to offer me a Firefox or Chrome plugin. And last time I tried they did not work in Opera because they suck and the moonlight devs still haven't got a clue what the web is all about.
Moonlight is even more broken than Flash (requires certain browsers, different plugin for different browsers, etc.). Nicely done, Moonlight crew!