Big Finish scripts tend to be more consistently good, because they're not constricted by having to appeal to casual TV viewers, so they have the freedom for more complex, darker writing, more ambitious and bizarre ideas, and concepts that span over a larger period, making for a more rewarding experience, as well as the return of characters and locations known by long-time Who fans without the fear of alienating others. As someone else said, the lack of a visual representation means the dialogue and story have to be on point, an advantage I think audio stories have over television in general.
It also helps that Big Finish have some of the most talented writers to have ever worked on Doctor Who.
