Why Tom opened the Chamber just once

Why Hagrid was allowed to stay; Why did Tom opened the Chamber just one time

'Only the Transfiguration teacher, Dumbledore, seemed to think Hagrid was innocent. He persuaded Dippet to keep Hagrid and train him as gamekeeper. Yes, I think Dumbledore might have guessed. Dumbledore never seemed to like me as much as the other teachers did...'
'I bet Dumbledore saw right through you,' said Harry, his teeth gritted.
'Well, he certainly kept an annoyingly close watch on me after Hagrid was expelled,' said Riddle carelessly. 'I knew it wouldn't be safe to open the Chamber again while I was still at school. But I wan't going to waste those long years I'd spent searching for it. I decided to leave behind a diary, preserving my sixteen-year-old self in its pages, so that one day, with luck, I would be able to lead another in my footsteps, and finish Slazar Slytherin's noble work.'

THE HEIR OF SLYTHERIN - Pg 329