UCSD Pascal was developed prior to the Apple II, during the 70's in San Diego, using PDP-11 class machines with a 512-byte block disk structure. In the process of porting it to microcomputers, often (not always) the file system also got ported.
UCSD Pascal was seen as a closed environment offering everything in one place. Today we might call this an IDE and see it as an application on top of an existing OS. But back in the very early microcomputer days most machines didn't have a DOS of their own. And even if some OSes were available, they were usually rather frugal and mostly just a bunch of hacks to attach floppies one way or another (let's be serious, the Apple DOS was one of them). Thus just only adapting the very basic hardware drivers while keeping everything above in transportable P-code, was a rather clever solution.
Keep in mind, the goal of the P-System was to offer students a standardized programming environment, where they could work on every machine, the university could afford, like on a PDP and have create exactly the same replicable results a course requies. It was never ment to be a language kit for software development for microcomputer systems.
To some extent it was quite similar to early mainframe applications — what we nowadays see as an OS was an integral part of the application, thus forming a world of its own. It wasn't until much later that the P-code system changed from a standalone application to a runtime package running on top of a 'regular' OS (like PC-DOS).
Adopting Pascal for the Apple II can be seen as the first major step of Apple into the educational market. In the course of history, SOS and later ProDOS did inherit the 512 byte block size from the UCSD P-code system, as Pascal was the main implementation language for the Apple III — and later all Mac software. In fact, for some time the Pascal development did also drive Apple II development in general. For example, release 1.1 introduced the 'Firmware Protocol' that required add-on cards to show certain signatures at $Cn05/07/0B and $Cn0C to help identify a card and its functions. Due to the importance of Apple Pascal, hardware manufacturers added them soon. ProDOS (and GS/OS) rely on these markers to identify and handle cards they know.