Bruce sat down with Longstaff and his parents after his contract had expired before clinching a deal following months of negotiations and a change of agent for the midfielder.
Bruce told Chronicle Live: "Deep down, the boy never wanted to go.
"I think people turned his head, which happens with young players.
"I think a conversation with his mam and dad, and him, got it over the line, which I'm delighted about."
United were lambasted by some fans when they allowed promising left-back Lewis Gibson leave to join Everton in 2019.
Bruce reflected: "It was important that we didn't lose our young players.
"Unfortunately, Matty's had that niggling thigh strain.
"So we hope he can get better."