Board Thread:The Panopticon/@comment-70.72.211.35-20130120171335/@comment-1432718-20130120222547

SmallerontheOutside said it best - if you don't know why an edit was reverted, ask. Admins often do a lot of mass reverting (either vandalism or someone unknowingly breaking a rule on dozens of pages) and the rollback feature is just more convenient for them. If you ask, they will answer, and you can learn something new about the wiki.

Making summaries mandatory would just cause a lot of headaches. Yes it's helpful, but some users do lots of edits and mandatory summaries would be burdensome.