Because you're not telling tar to append. You're overwriting the tape every time, starting at the beginning of the tape and writing towards the end. It's like overwriting the same file every time.
You're archiving with something like 'tar -cf /dev/rmt/0', yes? That creates a
new archive every time, overwriting the parts you've already saved to. 'tar -rf /dev/rmt/0' will append to an
existing archive. See 'man tar' for details.
here is a google groups thread describing the process.