Because gzip and bz2 exists. 7z is almost always a plugin or addon, or extra application. While the first two work out of the box pretty much everywhere. It also depends on frequency of access, frequency of addendum, size, type of data, etc. If you have an archive that you have to add new files frequently, 7z is gonna start grating on you with the compression times. But it is Ok if you are going to extract very frequently from an archive that will never change. While gz and bz2 are overall the “good enough at every use case” format.
Because gzip and bz2 exists. 7z is almost always a plugin or addon, or extra application. While the first two work out of the box pretty much everywhere. It also depends on frequency of access, frequency of addendum, size, type of data, etc. If you have an archive that you have to add new files frequently, 7z is gonna start grating on you with the compression times. But it is Ok if you are going to extract very frequently from an archive that will never change. While gz and bz2 are overall the “good enough at every use case” format.