Because people want to share their emotions. Ignoring someone is probably triggered by some sort of emotion. And you feel a need to share it. You can't put it on Instagram that you are ignoring someone on the DIS, that would be silly as your followers probably don't know what the DIS is.
Or maybe it is a sign of a feeling of superiority "some people are so stupid I cannot handle it and people have to know there are stupid people on here. If I find them stupid, others might think so to!" Although that last remark could mean they are actually very lonely and/or insecure and need a buddy to validate their thoughts.
I have one person on ignore, and I might add a second. There are also some people on here whose comments always amaze me, but I don't ignore them, because I feel superior every time I read their comments