For the same reasons, though on a very slightly lower scale. For very little gain, it causes a lot of confusion. Or rather would if Perl6 ever started being used and there was a chance to see and confuse Perl5 and Perl6 code.
I don't think Perl6 ever gains a reasonable sized comunity. Not because it's several years late, though that adds to the problem, but because it went completely overboard with special characters. While they "simplified" sigils (read: changed the way they worked for years) and changed the object-method "separator" to something that's kinda resembling the standard (again changing things that worked for years causing confusion), they also introduced lots of new line-noise operators and their combinations and twists.
If Perl5 looked like line noise to an unaccustomed eye, Perl6 looks like line noise even to those that love and use Perl for years.
Enoch was right!
Enjoy the last years of Rome.