Same reason as why Google has buried open standards like RSS or XMPP based GTalk.
Seriously I do not think that the stakeholders' values of Google are the same as the ones by the Mozilla fundation (which also has a corporation but mainly due to taxing and employing reasons).
A big drawback of Pufferfish.js is that images are not indexable by Google. That's not the case with ResponsivelyLazy.js. It works great in search engines, social networks and read-it-later tools.