It originally used a per-process model primarily because of the age of the codebase. It predates commodity SMP hardware (1986ish onwards, starting with Sequent) and standardized threading APIs (1993):
> DBMS code must run as a separate process from the application programs that access the database in order to provide data protection. The process structure can use one DBMS process per application program (i.e., a process-per-user model [STON81]) or one DBMS process for all application programs (i.e., a server model). The server model has many performance benefits (e.g., sharing of open file descriptors and buffers and optimized task switching and message send- ing overhead) in a large machine environment in which high performance is critical. However, this approach requires that a fairly complete special-purpose operating system be built. In constrast, the process-per-user model is simpler to implement but will not perform as well on most conventional operating systems. We decided after much soul searching to implement POSTGRES using a process-per-user model architecture because of our limited programming resources. POSTGRES is an ambitious undertaking and we believe the additional complexity introduced by the server architecture was not worth the additional risk of not getting the system running. Our current plan then is to implement POSTGRES as a process-per-user model on Unix 4.3 BSD.
> In POSTGRES they are run as subprocesses managed by the POSTMASTER. A last aspect of our design concerns the operating system process structure. Currently, POSTGRES runs as one process for each active user. This was done as an expedient to get a system operational as quickly as possible. We plan on converting POSTGRES to use lightweight processes available in the operating systems we are using. These include PRESTO for the Sequent Symmetry and threads in Version 4 of Sun/OS.
> DBMS code must run as a separate process from the application programs that access the database in order to provide data protection. The process structure can use one DBMS process per application program (i.e., a process-per-user model [STON81]) or one DBMS process for all application programs (i.e., a server model). The server model has many performance benefits (e.g., sharing of open file descriptors and buffers and optimized task switching and message send- ing overhead) in a large machine environment in which high performance is critical. However, this approach requires that a fairly complete special-purpose operating system be built. In constrast, the process-per-user model is simpler to implement but will not perform as well on most conventional operating systems. We decided after much soul searching to implement POSTGRES using a process-per-user model architecture because of our limited programming resources. POSTGRES is an ambitious undertaking and we believe the additional complexity introduced by the server architecture was not worth the additional risk of not getting the system running. Our current plan then is to implement POSTGRES as a process-per-user model on Unix 4.3 BSD.
(THE DESIGN OF POSTGRES, 1986, https://dsf.berkeley.edu/papers/ERL-M85-95.pdf )
> In POSTGRES they are run as subprocesses managed by the POSTMASTER. A last aspect of our design concerns the operating system process structure. Currently, POSTGRES runs as one process for each active user. This was done as an expedient to get a system operational as quickly as possible. We plan on converting POSTGRES to use lightweight processes available in the operating systems we are using. These include PRESTO for the Sequent Symmetry and threads in Version 4 of Sun/OS.
(The implementation of POSTGRES, 1990, https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.93... )