Earlier quoted context omitted.
SPDY was donated to the IETF for adoption into the HTTP/2 standard. Other browser vendors only implemented it at that time. WebP is a file format and doesn't fall into the normal standardization process. Browsers implement support for new file formats at their own behest.
Yes. After SPDY had already been implemented and shipped in Chrome.
You'll note that they're also willing to kill features which are not adopted, like PPAPI.