The speed is a factor, but not the main issue. Also there is not that appealing UI but that's not a major issue, just a minor observation. The main issue is that having JRE is not as simple. There are different packages that not all of them run the every java program properly. It is a hit an miss. For example we have OpenJDK, IceTea
That confusion and unclear errors about why each of them cannot run some java program is something that I want less in my day to day life.