The nfl has much bigger say over who plays on NBC than the NBA does on who plays on ABC.
The NFL also doesn't wait 2 full months before televising an NBC game....
The NBA's showcase games are TNT anyway. Those are the events held for the fans. ABC games are broadcast just because ABC owns ESPN and ESPN has the contract. ABC only wants the playoff ratings.
If ABC could opt not to show any nba regular season games outside of Christmas and just broadcast playoff/finals games, they would. They most likely agreed to show a certain number of regular season games on ABC, but demanded a say in what teams they would show.
San Antonio came within one shot of winning the title last season. They are on ABC once, and they only get that game because it's a finals rematch against the heat.