I think that the Linux kernel is reasonably secure, but it still suffers from frequent vulnerabilities.
But to answer your question, I think you see a lot of Android malware for two reasons:
1. Android devices aren't usually patched with the latest security fixes.
2. Most malware exploits the application frameworks that sit on top of the Linux kernel. These frameworks aren't nearly as secure. And if an application framework has a kernel-mode component where code can be executed? There goes most kernel security protections.