I didn't realize Postgres already had SQL support in 1994! I didn't try to use it around that time; msql was a thing for a while, and then people moved to the protocol-compatible MySQL. I was using INFORMIX in that job (1996–7) and it wasn't for several more years that e.g. Slashdot switched to SQL backends.
So why didn't people use SQL in Postgres95? The licensing was fine. My best guess is that the SQL support wasn't yet good enough to be useful. The database as such (transactions, data types, persistence) was already pretty solid, as I understand it.
Didn't those happen nearly at the same time? Postgres95 (which PostgreSQL started from) had support for some subset of SQL (including inner joins): https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f... outer joins were added a bit later: https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit...