To elaborate on Toby Allen's answer (this wouldn't fit into a comment)....
One of the main guys behind Suhosin is Stephen Esser. Stephen seems to have had on ongoing disagreement with the PHP core developers with regard to security over the last few years. He was also one of the guys behind the month of PHP bugs which was intended to draw attention to the (in Stephen's opinion) sad state of PHP core security.
Given that the Suhosin guys have decided to go their own way and work outside the PHP project, I can imagine that:
- It's possible that Suhosin hasn't been contributed back for inclusion.
- The Suhosin guys haven't been able to convince the PHP team of it's usefulness, or haven't tried.
- The core PHP team isn't open to contributions from the guys behind Suhosin.
Some Linux distributions such as Debian (Etch and Lenny), Ubuntu and Arch include the Suhosin patch in their PHP package, so on those systems you'll often find it's turned on by default. Red Hat derived distributions (Red Hat Enterprise, CentOS, Fedora, etc) don't include Suhosin in their PHP packages.
Note: I have no association with Core PHP devs, or Suhosin, but a reasonable guess based on some of the personalities involved.