With Nabble as we know it: the option could clash with the preferences of users.
The user's preference for a view is probably set in a cookie, and that preference applies to all uses of Nabble. IMHO this is a good thing: I like threaded view, and I would not like any forum to over-ride my preference.
There was a similar question last year,
Selectable Default View. I have not read that yet, I will do now…