- You could update an archive in place. Not incredibly useful, but certainly nifty.
- It had support for multi-disk archives. Again, big deal in the 90s.
Finally, last but not least: it existed in the PC world. Ports of software between UNIX and PC systems were rare back then, and were made more difficult by the fact that the software would have to run in a segmented (16-bit) x86 environment.
My older sibling exposed me to tar indirectly by granting me access to their university-provided dialup UNIX account where they ran `elm` for email access, and this happened long after I had been using BBSs for years with my PC.
Never saw a tar file in all my years of PC-based BBS usage. GNU/Linux and Freebsd are largely responsible for the general public accessing *NIX-like systems. BBSs (and their file sharing) were a thing before those existed.
XENIX is too often forgotten, as is the fact that the Unix world had its "archiver wars", too. In this case it was tar versus cpio, with the POSIX standardization effort settling on "pax", the pun in whose name is reportedly not a coincidence.
There was for a good while a bit of a "compression arms race" with folks looking to find the most optimal compression to save precious time on BBS dial-up. For a while 'ZIP' won, then 'ARJ' managed to squeeze out a bit more, and so on -- though I think diminishing returns kept things from going too far beyond ZIP/ARJ on many BBSes.