There are so many hours in the day. Developer Rare is not going to widdle them away making downloadable content for Viva Piñata. Rare is doing other stuff, 'kay? A member of Rare's team commented on its website:

We aren't doing any downloadable content for Viva Piñata because we are much too busy doing something else.

Yeah, something more important. The game didn't turn out to be the runaway smash hit Microsoft was hoping for. But like many things Xbox 360, Microsoft will push on valently with Viva Piñata by releasing PC and DS versions. All is not yet lost.
Rare Says No More [Games Industry]