TL;DR: Because, standards.
One of the biggest pain point among front-end developers is cross-browser compatibility; and nothing would get them more enraged than seeing a browser deviate from a standard. If I had a nickel for every time I heard @alkshendra and @fazlerocks curse IE <9, I would have amassed quite a good amount of fortune. :)
As @jiyinyiyong pointed it out, a browser doing things "uniquely" is a big no; it fragments the concept of "open" web. If one browser does it, all browsers should fall in line, and it would mean the creation of a new standard, which is a long and painful process. On a related note, see @leaverou's answer to Why does it take so long for new specs to get browser adoption?
Google did propose an addition of support for multiple VMs into Webkit. One of the comments in the thread hits it spot on:
Previous branches have been used to bring up interestingly complicated features, or features that had the potential to cause dramatic stability issues during their early work (such as the old svg-experimental branch). This project appear to be largely a make work project as it's already possible to have bindings for multiple languages (as the C++, GLib, ObjC, V8 and JSC bindings demonstrate).
It seems an academic exercise to see if we can create a general architecture to make more bindings, as is exporting support for proprietary extensions like vbscript, python or dart to the web. As the 90s demonstrated such "features" are bad for developers, and bad for the open web. This may not be apparent to people who have not spent years working in the environment but random additions frequently cause significant pain down the road, even in the cases where the overall result was a "good" thing -- such as canvas - for the subsequent standardisation caused us quite a bit of compatibility problems, even though it was a very compact and contained api.
— Oliver