Because I usually need a good reason to learn a new language, something to use it for, something it does better than whichever language(s) I'm currently using. Despite all the "there aren't that many changes, honest" claims, Perl6 seems to have enough subtle changes that one needs to treat it like a new language. I've been watching (and sometimes commenting) the #perl6 channel for quite a while, that's how I come to this conclusion. If I, at least, try to treat it like "just another new perl version" I'm sure I'll get into great tangles trying to remember which syntax is p5 and which is p6.
I'm aware that p5 had quite a few changes over p4, but weren't those mostly additions (eg modules), rather than actual changes of significant syntax?