In a recent letters column on their website Rare says that they're too busy working on other titles to develop any more downloadable content for Viva Pinata:

[Q:] If you do make a proper VP sequel, will there happen to be Jelis, Pengums, Polollybears, a Kangaroo piñata, a Leminee, a Giraffe piñata, and any other Cryptids that have been referenced before? Is there the possibility that instead, those piñatas will be downloadable through Xbox Live?

[A:] And: "We aren't doing any downloadable content for Viva Piñata because we are much too busy doing something else..."

Too busy developing more games or too busy to waste any more money on a failed title? Ah, well...there's always Viva Pinata DS.

via GI.biz