Carter Bryant didn't play in San Antonio's final Summer League game for good reason. While there was no actual reason put on the record, it's clear why they sat him. They saw everything they needed to see. Those summertime exhibition games are less important than the random flashback episode nobody asked for on your favorite TV show.
Bryant was drafted to play defense and knock down open shots. He took to the defensive side of the ball immediately, but the shotmaking was lacking. It's obviously nothing to worry about long-term, but it was so bad that it's understandable if the Spurs wanted to see if he could quickly shake off those struggles and show some early poise.