Simplicity is useful

if you assume that privilage escalation exploits are everywhere, nothing less than a fully locked down SELinux system can do you any good (and note that _no_ distro is shipping a _fully_ locked down SELinux system)

if privilage escalation exploits are not everywhere then chroot is much stronger.

and even though it's not as strong as other security mechanisms could be, the fact that those other mechanisms aren't used makes them pretty useless

however, I will disagree slightly with chroot being the most successful model, I'll point out that it builds on the basic unix user/group permissions, and I would call _that_ the most successful model