Speaking Wednesday on the SEC coaches' weekly teleconference, Stoops said the junior running back was frustrated with his diminished role through four games this season.
"As you know, he's had only a few carries here this fall," the UK boss said. "So, yeah, we did see it coming. It's one of those situations where, for whatever reason, he gets behind on the depth chart and has to earn the trust of the coaches and the players as he moves forward. He was starting to do that and doing a better job and being more accountable, and then some other guys emerged."
Horton's decision to leave the program was first reported by UK blog Kentucky Sports Radio.
Kentucky (2-2, 1-1 SEC) has run the ball well this season, averaging 196.8 yards per game. Junior tailback Boom Williams is averaging 8.0 yards per carry, while emerging freshman standout Benny Snell Jr. is averaging 6.3 yards per carry and leads the team with five rushing touchdowns.
Horton, a former three-star prospect from West Chester (Ohio) Lakota West High School, had only three carries for 12 yards this season. He had shown promise earlier in his career, though, rushing for 109 yards on 14 carries last season against Tennessee.
"There's only one football, and we've been playing pretty solid at the running back position, and he had a hard time getting back on the field," Stoops said. "So we wish him the best of luck, but depth is certainly not an issue. We've got quite a few backs on scholarship."
The Wildcats also feature senior Jojo Kemp, sophomore Sihiem King and freshman A.J. Rose as scholarship tailbacks.
Horton is now one of 10 players from UK's highly-touted 2014 recruiting class to leave the program. Seven of those players were from Ohio.
Kentucky travels to Alabama on Saturday to face the No. 1 Crimson Tide (4-0, 1-0 SEC), who rank No. 5 nationally in rushing defense.