(purely opinion based)
Because browser developers are different then language creators.
A language creator wants their developers to be happy and have the latest and greatest because only developers use languages and so the target demographic is pretty set.
A browser developer has 2 target demographics - the user, using the browser, and the developer, developing websites that need to work on multiple browsers. In this case, the user base is infinitely bigger then the developer base and users don't care about Flex grid or WebP images or etc... They care about pages rendering fast, or security, or plugins or sharing bookmarks across devices.
So while a php developer and say someone that only does front-end are pretty much equally important now a days, it's easier to give the php developer what he / she wants as theres only a few of them (relatively) compared to the incredibly more people just using the browser and not developing websites.
</priorities>