Steemit isn't as big as Reddit, so less people are gonna be commenting on the comment sections.
And then there's a complete difference regarding Steemit, namely the economic incentive.
This reward might motivate people to be polite and to avoid shitposts and offensive comments, which might not be rewarded.
Another reason might be Steemit's lack of an app, on Reddit you can comment with your smartphone on the go, i think it is more difficult here