Well, GNU grep was last released 16 months ago, and the last change to its master branch was 4 weeks ago: http://git.savannah.gnu.org/cgit/grep.git
FreeBSD's grep was last updated back in August: https://github.com/freebsd/freebsd/tree/master/usr.bin/grep
OpenBSD's grep was last updated 11 months ago: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/grep/
Oddly, it looks like the Darwin grep was last updated in 2012: https://opensource.apple.com/source/text_cmds/text_cmds-99/g...
Strange that Apple would be shipping such an ancient grep.
I don't think it is that strange. Command line tools such as grep don't appear to be a development priority for Apple. Their focus appears to be on features visible to the average user, who uses the GUI instead of the command line.
Command line tools are mainly used by developers and power users, and the existing tools are generally good enough for most purposes, and people who want something better can always install the GNU versions using Homebrew/MacPorts/etc. There isn't much market demand for improvements in this area, so it makes sense Apple wouldn't invest in it.