It turns out, they did. Camalot Software Planning, makers of Mario Golf, made We Love Golf for the Wii. However why Capcom published this, and why this didn't become a mainline Mario Golf game is anyones guess. Maybe after Gamecube, Nintendo just didn't have too much money to publish a game like this?
Camalot Software Planning (makers of Mario Golf/Tennis) barely made any games during the last 5 years. In the 6 years between 1999-2005 Camalot Software Planning made 9 games. In the last 7 years, they made 3 games, and ported Mario Tennis for the Wii.
So I don't know what happened to Camalot, they used to publish a game a year, and then they suddenly dropped to a game every 2 years? I think just after Gamecube, Nintendo didn't have much money, and it took them this long to revamp Camalot. Mario Tennis did moderatly well for a handheld Mario Tennis game, if it has legs it could be the best selling portable iteration. I think Mario Golf will do the same when it releases. I think it would be wise for Nintendo to at least make a Mario Tennis game for the Wii U.
This could have sold like hot cakes on the Wii, considering how popular Wii Sports - Tennis was.