InvoicePlane on centOS 6.5

As you know, centOS has PHP 5.3.3 installed by default and Red Hat backports security fixes and important bug fixes to all packages they offer in their repositories until the EOL of that particular RHEL version, PHP included (last update was 6 days ago).
So why 5.3.3 is no longer supported? Security? or new maj features?

Both security and compatibility reasons. Not even PHP 5.5 is supported anymore.
Instead of trying to be compatible with unsupported, old systems we chose to push forward and adopt newer versions as fast as possible. Except for compatibility there is not a single reason to use such an old PHP version and I highly disadvise to use any version below PHP 5.6 even if some third party vendor is offering updates.