I'm just lazy... ♥
Unless the manga was astounding or something, or just very shocking, then I might comment. And that only happened once so far. xD

But I do rate them though, only one click! : D

Time to put a rating: 1 second
Time to write up a decent comment: A couple minutes

That's why I rate more than I comment

"It is those who know little, and not those who know much, who so positively assert that this or that problem will never be solved by science."

I try to give a comment on each manga i've read, but some end up getting lost in the shuffle.
In the end it depends on what time do i have.

As for me, it's because I prefer discussing it. If I know no one can reply to my comment, I get bored.