Garza doesn't play much for Minnesota because Rudy Gobert has been in the NBA since 2013/14 season, is 7-1 and weights 260lbs, and is being paid 41 million dollars this season, Naz Reid has been in the league for 4 years and is being paid nearly 13 million dollars this season, if you want the reason why Luka Garza doesn't get any playing time it's because of the money the organization is paying Rudy and Naz. It's all because of the money and has nothing to do with how Luka plays defense. Who in the NBA plays defense anyway. Heck, if you just look at Gobert, Reid, and Garza, Garza is absolutely the better offensive player of the three, but that's the NBA, lot's of players get lost in the shuffle because of the stupid amount of money this players are paid.
The Timberwolves are also in first place in the Northwest division, they have no reason to make any roster changes any time soon.