Making comments on people posts actually shows that you've really read such person posts comprehensively and you may decide to either make positive comments by commending such people for coming up with such great post or you make a negative comment which isn't really advisable maybe because you aren't happy or pleased with such posts. Making comments on posts also give rise to the asking of questions, criticism and positive commendation to people posts just like you may also intend to have the same.
From my little research on the Steemit platform, I think some people don't actually comment more on Steemit as a result of the below three reasons ;
1) LACK OF UPVOTES: The major reasons some whales tend to have more comments is as a result of their compassionate upvote rewards or generosity towards people who make comments on their posts. I am sure you won't want to miss that great reward or opportunity also. This explains the reasons we have more people making comments on whales posts and other influential Steemians with high SP.
2) LACK OF QUALITY CONTENT: How do you expect people to make comment on post that has no specific quality which can't catch the audience? This explains why making quality posts is extremely important on the Steemit platform. You need to know your audience and catch them with your posts such that they keep asking for more. Making quality posts will surely give you more comments on your Steemit blog and the lack of quality posts can lead to negative comments or no comments on your posts.
3) POST VISIBILITY: The more the number of people who can view your post on the Steemit platform, the more chance of you earning more comments provided your post is attractive and contains the required spices and quality. This is one of the reasons most people make use of resteem bots and other resteem services on the Steemit platform to enable more users read their post which leads to great increase in the number of comments.
In conclusion, if you really want to make people commenting on your post then you need to start rewarding people who make comments with cool upvotes, ensure your post catch the audience by making quality posts and lastly, always ensure you make your posts more visible to other Steemians.
Thanks for reading and I hope this helps.