The poot exploit is limited by a few things. First, it is not in the base Android code from google. The bug was added in later by a company who then passed out their modified Android to companies like LG, HTC, Samsung. Second, just because the kernel source contains the buggy code doesn't mean that the code is being used. And even if the buggy code is being used, it may not be accessible to all the different user-level apps.
It just happened that on the Optimus Elite, they used a configuration that exposed the buggy code to all the user-level apps and we could take advantage of it.