Why? well the reasons are complex but to put it simply...
GZip Tar Archives are smaller.
For the complex reasons why this is so I refer you to "GZip Tar - vs - Zip, Archives".
Disclaimer: I am NOT a Windows user, and never really likes the restricted enviorment provided by it. I prefer the more free form environment of UNIX and X Windows, and now Linux. I use to do my work from a Sun Color Desktop Workstation, but have now moved to SuSE Linux, then later to Redhat Linux running on a PC with huge 1600x120 pixel display.PC users can still de-archive the "Gzip'ed Tar" (.tar.gz or .tgz suffixes) by using the following archive programs, or getting the source to both the GZip compression program and a Tar Archive Program from...If any of the information provided, is wrong, incorrect, or you would like to add another choice, mail me. Otherwise I do not really want to know about PCs and their problems. ;-)