Why Centos still not using Latest kernel

CentOS 6 is based on RHEL 6, which was released in 2010. Within a major release, the goal is compatibility and not introducing breaking changes as the target audience is enterprises that value stability over features. As you can see from the version history, they don't introduce new kernel versions within a major release (but some fixes are backported), so it's likely RHEL 6/CentOS 6 will stay on 2.6.32 (a stable kernel version).

To get a newer kernel, you'll have to wait for RHEL 7/CentOS 7 and upgrade. It will include kernel 3.10 (another stable kernel release).

Since Red Hat supports each release for 10 years (or 13 with extended support, e.g. RHEL 6 until 2023), it's no surprise they don't release a new major version every year.