That would probably make the system more susceptible to gaming by spammers.
How does forcing registration help? The main barrier is that you need to answer a couple of questions and get upvotes before you can upvote others.
SO probably doesn't want too many unregistered users floating around. Also, one of success metrics for an Q&A site is the number of registered users. So, it does make sense to force users to go through almost painless process of registering before they really start engaging in the site.
I'm guessing (don't know for sure here) its an anti-spam measure...