Because people used it for some improper things. Specially for commercial purpose and so. Links too.
To be able to achieve that, the user that might attempt on doing that should be a senior member(more than 120 posts i guess).....
And, no fool would waste his/her time posting unreasonable/nonsense reply if he/she just want to post links,advertisements and etc....
That is possible, there are remote users that could still do that.... that's why adding more phrases on a signature is not allowed.....