- tar has been around forever
- tar is standard on pretty much every *nix system (which GNU tar
becoming more common even on commercial Unices)
Tar is easily available even on Windows. Good programs like 7-zip and many, many others, can handle tar well.
- gzip provides better compression than zip (bzip2 is even better but
it takes lots of CPU)
rzip, which is built on top of bzip2, manages to compress significantly better than bzip2 (specially for large files), while being significantly faster.
A drawback is that it cannot work as a filter (IIRC, it can't read from a pipe, and the author says that the design of the algorithm makes it hard or even impossible to make it able to read from a pipe).
It was done by Andrew Tridgell (which dispenses presentation).
7zip compresses even more (more than zip, gzip, bzip2 or even rzip) but is very slow.
My choice is rzip.
I really don't know why isn't rzip integrated with tar (like gzip and bzip2 are) and why isn't it more widely used.