Familiarity, lack of pain, not convinced the alternatives are sufficiently better to justify the learning curve, etc.
In short, Eclipse mostly works for what I do, and while it frustrates me at times, there isn't enough pain there to motivate me to throw away the investment I've made in learning to use, configure, and customize Eclipse - for an alternative which may or may not even be better. Also, Intellij didn't always have the Open Source version, and I generally avoid using any software that isn't F/OSS. So for the longest time, my only real options for a Java based IDE were Eclipse or Netbeans. I played with Netbeans a little here and there, but never found it compelling. shrug
In short, Eclipse mostly works for what I do, and while it frustrates me at times, there isn't enough pain there to motivate me to throw away the investment I've made in learning to use, configure, and customize Eclipse - for an alternative which may or may not even be better. Also, Intellij didn't always have the Open Source version, and I generally avoid using any software that isn't F/OSS. So for the longest time, my only real options for a Java based IDE were Eclipse or Netbeans. I played with Netbeans a little here and there, but never found it compelling. shrug