Earlier quoted context omitted.
Don't think there's a technical reason. I'd previously seen this site linked from here that does it slightly more traditionally: https://regexcrossword.com/
Lost interest on 3rd beginner puzzle when validation failed for multiple valid solutions.
As far as I can tell, the puzzle is fully constrained.
From the start, the bottom-left cell as well as the right column can be solved just from the starting hints, and then you can derive the top-left cell per the backreference.
In particular, you can reframe the right column's hint as (AB|OO|OR), and only one of those satisfies the bottom row's hint.