Because it is as much a goodbye as any :P
The Brigadier and Sergeant Benton
It had been so odd to have been anywhere inside the TARDIS with the Brigadier and Benton along for the ride. But in a way, he had been glad that both of them had shown up. With the help of Bessie, they had been able to get the two civilians and Jo to the safety of the UNIT headquarters which had also been transported to the...nowhere land of anti-matter on the other side of a black hole.
They hadn't been too much help in defeating Omega, but his other selves had been. And Jo shouldn't have been there at all, the silly girl.
Still, life would go on for both the Brigadier and Benton. Leaving them behind, and having gotten his memories back on how to fix his TARDIS (and a new Dematerialisation Circuit along with the information) he had been all too happy to see the back end of Earth for a while. On his own terms and not those of his rather annoying people.
He knew though that he'd go back.
For one, it was where Jo belonged, and one day she'd want to go home. For another...he found himself actually liking them both, regardless of the militaristic ways they shared.
Benton wasn't half as bad as the Brigadier on that count, and had a pretty good head on his shoulders when he wanted to use it. The Brigadier just liked blowing things up, or so it felt most of the time.
Though he had tried his best to stay apart from everyone, to not make connections that would all too soon end, he found that the UNIT crew were like a secondary family to him. In some ways, he was closer to the Brigadier's army and men then he had ever been with his own family.
He just hoped he didn't get yelled at when he came back with Jo the next time they stopped by for a visit. He had begun to forget how dangerous the universe can be.