[LoneTech]

I wouldn't mind zip much, but rar is a non-free format to the point that I know of no free creator for it. It's popular for the simple reason that it compresses well, but that's a bit of a marginal factor here.

Zip isn't actually supported by default on most unix-like systems, but fairly easily installable (typically the info-zip implementation).

Tar and gzip (separate stages, as you noticed extracting one at a time) usually are, except on Windows. Still, it's fairly easy to install there, as 7-zip and a bunch of other programs support it. The non-free programs WinZip and WinRAR both do.

If you want a more level playground, 7z archives would force everybody to look up 7-zip, which is both free and available to each platform. It's growing in popularity as it compresses quite well. Compared to rar, I guess its weakness is that it doesn't have a redundant multi-part feature (where you can lose a part and still get your data), but you can do that with parchive. It's rarely needed, anyway, but made sense when dealing with floppies.

In short, I would suggest the reason is likely habit; we always have tar. Making zips requires an extra installation step.

Addendum: It would appear ExtractNow also supports tar.gz, but from some sort of messy-DOS tradition doesn't recognize this extension. It might do the right thing if you rename the .tar.gz to .tgz instead, and your .tar.g-0 may be a misnamed .tar.