I was wondering why docx format would be chosen instead of PDF but they answer it pretty completely here if anyone else is interested: https://www.uspto.gov/patents/docx
It looks like most of those translate to "We build our automated systems around DOCX so you get all our features if you use it". But it doesn't really say why they chose to build on docx.
Having worked directly with their teams in the past (although not on this), a lot of their systems seemed to evolve naturally over time based on the needs present. In that industry, a large majority of the documents being passed back and forth are DOCX. So my semi-educated guess is someone built a system to handle some simple intake tasks for DOCX applications because a large majority already were, it evolved over a few years, and when they finally decided to fully automate the process, they decided to build upon what they have which only supported DOCX and it was cheaper/easier to mandate everyone submit in that format than to build a new system or add support for others.