Because spaces are rendered the same everywhere. Suppose I write this code, indenting using spaces.

    im_a_long_method(argument_no_1,
                     set_indentation_true(...),
                     no_string_cutoff,
                     whatever)

Now suppose I used tabs to indent and my editors tab width is 4 spaces while your is 8 spaces, then the above would look like:

    im_a_long_method(argument_no_1,
                                      set_indentation_true(...),
                                      no_string_cutoff,
                                      whatever)

Since not every editor agrees on how wide a tab character is, you get crappy alignments. Therefore use spaces.