It's because WiiSpeak just came out, and WM+ Isn't out yet. In case you forgot, calling your dog by his/her name was a very important feature (although, I personally could not use it do to puberty's rapid progression of my vocal chords), and without WM+, throwing a ball or a disc (very important, and in WiiSports resort), wouldn't work nearly as well...
Nintendogs Wii will probably be out by the end of 2009 or Mid 2010, end of 2010, tops.