Our current setup means users must receive an up-vote to post on walls (or ask five questions and select five answers). This is not always simple to achieve, especially on the Meta and RMT sections. Changing the requirement to 22 would mean users can access wall posting by asking a question then selecting an answer, which is easier for them to do on their own.
Reading between the lines, I think 30 points was chosen over 22 points to reduce susceptibility to spam. I can say through years of experience that 22 points is impossible for spammers to navigate as long as the approval system is in place and it sends edited posts back to the queue (which it does).
Aside: it would be good to simplify the privileges system overall: