One reason, reasonable or not, is that indentation based syntax overloads white space in ways that muddle it's use for communicating with humans with it's use for communicating with machines. I have some sympathy for the argument (though it may be misplaced) because my gut suggests that the more a language relies on whitespace, the more human communication via formatting (such as reducing line lengths) becomes a corner case requiring special syntax and using that special syntax induces cognitive load when reading and writing the code.
That's not to say that for a Domain Specific language like HAML, the whitespace approach isn't useful. But that's down closer to declarative programming than trying to wire up a control flow.
YMMV.