Because 2010aa would come before 2010z in alphabetical order.
I knew there was a reason, thank you.
That has the flaw of conflicting with the ISO scheme for naming months. If we want to go that way, we should probably just go all the way and append a full date to each release.
Hm, and three digits would conflict with day of year naming, and four digits with year naming, ... Ugh. What a morass.
So in a hypothetical future where we need more than 52 release identifiers, the 53rd would be "zza", the 79th "zzza"? I don't know how generally the rule is described in the relevant documents -- if it doesn't say what to do beyond "zz", we could well end up in the same kind of trouble again...