[Orgmode] no syntax defined for comments

[andrea]

I sometimes want to write comments in my org-mode files, but the handy
function *comment-region* doesn't work, "no syntax defined for.."
Then I can choose '#' but it doesn't put it in the beginning of the
line.

I normally end in selecting the rectangle and run *string-rectangle*
with '#', but that's a bit annoying.

Why we can't just set '#' as the comment character so that normal
functions work fine?
Thanks