The main reasons are: Price and opportunity cost. My current D70 does great work, largest so far 20x30" and planned 30x45". I don't use high ISO. Where available I prefer a more portable camera.
Actually I find film ie., the $30 or $50 F/N75 great - it does AFS VR and same pro film and pro lab as a F6! Its FF too.
My only camera is the D70 and had it since 2004 and clocked up 4,000 images. In the near future I would like more travel so photog is a back step for that. But I would like a D50 or a D2h classic used in a year or two. In the nearer time, maybe a set of used 3x studio lights and a background or two for family portraits and experience that....
A used FF digital of course, I would like one but I don't need one and doubt what I see in the print of what I do is significant. Ie., for me its not worth it.
Esp that in digital they just get better and better and cheaper and cheaper. Esp those 2nd hand deals.