The 25-year-old joined Bournemouth last month on an initial loan deal until January, when he will sign a three-and-a-half-year contract.
The left-back is regarded by many as one of the best in League One and Slade told Sky Sports that the former Tottenham youngster could have made the step-up to the Championship next year instead.
"With all due respect, we were very keen to keep Charlie with us to be honest and thought that the player, his representatives felt that Bournemouth would be a bit of a sideways move really," he said.
"I could understand it if a Nottingham Forest or another Championship club came in for him because that is probably the move the boy was looking for and that certainly has been the area where interest had been.
"We knew that we would have a difficult job on our hands to keep Charlie here, that's for sure, but we would dearly have liked him to stay because he was a very important player for us.
"Well, he's 25 now and you'd probably see this as being the right time, if there was ever going to be one, to break through into the next level and I think that he can because he is a very good player.
"He is still improving though and he can only get better and I'm sure he will go on to play at the highest level, but it would have been nice to do that here with Leyton Orient."