My guess would be lack of time. Think of it this way, if artist is popular he has lots of wanted and unwanted attention, lots of people who want to commission and also those who are looking to steal their works for example. They get lots of feedback too which is physically impossible to manage. And there are always "iconoclastic" deviants out there, namely people who are looking to pull down some monuments by complaining about what they don't see eye to eye with an artist: style, theme, etc. It can be very draining if you pay way too much attention to what every single commenter says especially if what they say is based on their opinion not with factual things.