Here's a list of the games Camelot released between GS1 and DD: (Source: Camelot Software Planning's Wikipedia page)
~Golden Sun — 2001 (Game Boy Advance)
Mobile Golf - 2001 (Game Boy Color)
~Golden Sun: The Lost Age — 2003 (Game Boy Advance)
Mario Golf: Toadstool Tour — 2003 (Nintendo GameCube)
Mario Golf: Advance Tour — 2004 (Game Boy Advance)
Mario Power Tennis — 2004 (Nintendo GameCube)
Mario Tennis: Power Tour — 2005 (Game Boy Advance)
We Love Golf! — 2007 (Wii)
New Play Control! Mario Power Tennis - 2008 (Wii)
~Golden Sun: Dark Dawn - 2010 (DS)
Notice the roughly 4 year gap between the release of Mario Power Tennis and GSDD. That's how much time they had to make DD. Since DD was their first DS game, I'm going to assume they had to spend lots of time learning the ins and outs of DS development before even starting the engine, and then making the engine would have taken longer than other studios with previous DS experience as of 2008 or '09.
Fortunately, there's another entry in that list that gives me some measure of hope:
Mario Tennis Open - 2012 (3DS)
Perhaps, when it does come time for Camelot to make GS4, they won't be forced to ### around learning about the system before rushing the game. Hence, a less rushed game.