Though not explicitly stated, the deal is likely a reward for Matheny taking the Redbirds to the World Series in his second year in charge. Matheny's original contract with the club was set to expire at the end of 2014, but now he has a bit more breathing room so far as job security is concerned.
The former catcher had no prior managing experience when he took over for Tony LaRussa in 2012, though you wouldn't know it by looking at his early results. The youngest manager in baseball at the time of his hiring, Matheny owns a 185-139 record in his two seasons with St. Louis thus far -- reaching the playoffs in both years -- and has already come within two wins of bringing home a World Series title.
Though it's somewhat of a fool's errand to predict future success based on past results, the Cardinals' shockingly deep farm system and their already solid roster seems to portend more good things for Matheny in the coming years.