It's because "Ratzinger" is easier to spell than "Wojtyla". If we start refering him as Pope "B16", then others wouldn't have a problem with spelling. For example, we all know who we refer to with "Pope JP2".
Please don't take what I said too seriously. ;-)
"B16" sounds like a vitamin.
"Is your Church worn down? Tired? Just can't shake those spiritual blahs? Try a weekly dose of B-16 and feel your Church grow STRONG!"