|laziness, impatience, and hubris|
I'm not going to argue the logic of this. Being able to create a brand new language without making my jaw tired, running out of wire and having to go back to the store to get more, and in a non-smoking city ... that is great.
The fact is that most of the more popular languages use those parens. Whether your history is C/C++, Java, Perl5, or others which I've probably forgotten (I don't remember enough Pascal or Fortran to print out "hello world" - whether they do or not, I can't recall, but I'm not sure they quite qualify among the more popular languages, at least not from the perspective of those who would use Perl6), you're used to it. While it may not be intuitively natural for a human being to put parens around their conditional, it has been trained into a large majority of programmers to do it, even if that is against our nature.
Sometimes, bad choices are propogated to get over the inertia of the results of those choices. Rather than spending time retraining in the revolutionary new pattern of behaviour (I'd use the word "paradigm", but I think that word has lost all meaning), we stick to the tried-and-true-more-or-less. It helps languages, such as Java or C#, take off, because there is less retraining involved.
The trade-off here is that this may indeed be a revolution in language design. But it will delay the adoption of Perl6 (I think I've read something in the Apocalypses where this has been conceded already), possibly (although hopefully not) to the point where it just cannot acheive critical mass.
Another problem is for those of us who have to deal with multiple languages. I'm doing mostly perl5 right now, but I am still responsible for some C++ code (a couple KLOC), some shell scripts (probably about a dozen KLOC - ugh!), and have input into some Java code design issues. Yet another pattern will mess me up for a while - I can imagine that a number of people will just be too busy to pick up another language, especially one that has not (yet) hit critical mass, making it just a bit more difficult to acheive that critical mass. I'm not sure if I'll fall into this group or not yet.
I'm not saying that perl6 in general (or making parens optional in specific) is a bad idea. Someone has to come up with the revolutions every once in a while if we're ever going to get to that AI which just does what we ask it, without having to do all the drudgery of actually coding. It's just a risk. Whether good or bad, only time will tell.
Perl6 has, from what little I've read, so many time savers here and there - taken one at a time, most are pretty minor. Taken all together, it's pretty daunting. In some ways, renaming it away from "perl" may have been a good idea, so that no one thinks about it like perl. ;-) I think that perl6 shares less in common with perl5 than C++ shares with C ;-)