I don't know what the situation is now, but back when they were trying to push it toward standardization, NaCl was tightly integrated into the Chromium codebase with spotty public documentation. It would have been quite burdensome for other browsers to integrate and maintain, let alone for non-Google engineers to significantly contribute to and influence the direction of development.

That doesn't change the fact that there are web sites build with such technologies, which weren't adopted by other vendors.