Lowering the requirement for commenting has been extensively discussed in the past, and declined: Lower the amount of reputation needed to comment. You can read the reasons given (pro and con) in that thread.
A more sophisticated proposals of this kind is currently under discussion: Allow anyone to comment but hide low-rep users' comments until reviewed.
Without trying to replicate all the points made above, I'll give one. Questions and answers undergo review in multiple ways: by being on the front page and by being placed in review queues when applicable. Comments can be left on posts without anyone, other than the post owner, noticing. For many posts, the owner is no longer around: e.g., one could easily add spam or worthless comments to 3329 answers by Arturo Magidin. Thus, allowing a user to comment is putting an amount of trust in the user. And trust must be earned.