Lots of things. More specifically for Chrome, finding every function called to the 2.3.6 onward framework, adding it into our framework, adding the ICS holo light theme to GB 2.3.6 to put it in the simplest terms possible. Even if some were to take on those huge tasks, you have to remember that there may be functions in the new framework that are completely non-existent to previous frameworks, not just renamed or updated. It's a lot more complicated than copy pasting new functions into old frameworks. Even after all functions are brought back, what of the old ones that rely on the framework's old structure? You may be breaking everything to make Chrome work. It is complete trial and error. The more Chrome relies on ICS, the more and more it would be required to bring backwards compatibility to it. That's not to say it's impossible, but, it's much more favorable to wait for Google themselves to bring compatibility. It does sport a "beta" tag, after all.