Zip compression has the ability to break into parts for quite a long time already, so I don't think people are choosing RAR because of that.
RAR does have better compression, but only slightly, and if that is the winning advantage, everybody will be flocking to 7z instead.
IMO, the real reasons why I think RAR is the dominant format now among (coughs-ahems) certain users, are because of a few perceptions:
- You are a n00b if you use ZIP, and if you use 7z you are too hardcore.
- It identifies the n00bs when they start questioning how to "un-zip" the files, or what program to use for those "silly RAR files".
- WinRAR has a better interface with customizable icons, etc. Oooh shiny!
- You have to pay for WinRAR, and true coughs-ahems users will have a copy somewhere - it's like an exclusive club of sorts, and WinRAR is the membership card.
Welcome to the club, mate.