Time constraints keep me away from commenting.
I have difficulties writing short comments. Often, there is a lot I want to say, but time and concentration are not here, and there is the added difficulty of writing in English - I'm chronically exhausted and exhaustion and foreign languages don't work well together.
Sometimes I will try to at least give a small comment. But sometimes, the frustration of not being able to comment in more length is so much that I prefer not commenting at all.
Sometimes, I'm also afraid that if I write a long comment and then nothing for weeks/months, it may look like I grew disappointed/ disinterested in the work, when it actually means I respect the work so much I don't want to make a casual, works-for-anything comment, and that I have no time for a better one.