I actually question whether the real story is the fall of Eclipse or the rise of Intellij. My impression (as someone who used both) is that Intellij IDEA was significantly better than Eclipse (to the extent that I went out of my way to be the sole developer who used it on a team of Eclipse users) but that it was not widely used. Then they decided to release the community edition which enabled people to try it without…
Why couldn't a collective fork the old version, similar to MariaDB's reaction to Oracle's lawyer romps.
I wouldn't choose to aid such a project, UNLESS I felt that JetBrains (the company that produces IntelliJ) was being harmful to the community of users.
A perfect example would be OpenOffice. While it was well-run under Sun I thought most effort should go to a single project not to works. Once it turned evil (Oracle) I thought all support should go to LibreOffice.