Because Matt V was accessible and affordable. He was announcing the Padres at the time he was hired, if I'm not mistaken. Therefore he was right in the same area as the studios. Not to mention he probably costs a lot less then if you were going to go after some of the bigger guys like the two you mentioned above. Quite honestly I think Joe Buck would be a complete waste of money.
In order for Baseball commentary to work you need two guys who interact back and forth with stories, experiences, player and team information, etc. That's really the only workaround for developing great Baseball commentary. It's the hardest sports game to develop commentary for because there isn't action every minute, so you need to fill the non action with exciting stories, stats, previous game information, etc. That's a lot of stuff to record and place in a game to make sure it's not too repetitive.
Good post bwiggy. Commentary would be greatly enhanced with the addition of relevant stats vetting the current status of the game.