Everton have signed Keane, 24, on a five-year deal from Burnley.
The fee could rise to a cub-record equalling £30m and takes Everton's spending to almost £100m already this summer.
He joined the Clarets from United for just £2m in January 2015.
And the defender, who made his England debut against Germany earlier this year, was being linked with a return to Old Trafford.
But Keane reckons Goodison Park is the ideal place for him to develop as a player.
He told Sky Sports: "Of course it was a temptation [to join Manchester United] - it's your boyhood club but you have got to take your heart out of it sometimes and think what's best for you, and I feel as if this is the best place for me to come and continue my development.
"It's a club with such big history and a great fan base so I can't wait to pull on the shirt and play for this club."
Keane also revealed that Toffees boss Ronald Koeman, himself a former centre-half, played a big part in his decision.
"He used to play in my position," he added.
"I have been fortunate enough that in the clubs I have played in the past, the manager has played in my position as well and I feel like he was a very classy centre-half so he can still teach me things.
"I want to develop, I am only 24 years old, which is young for a centre-half but I feel how much they wanted me was a big factor as well.
"Bill Kenwright has got a great relationship with my representatives and he was a major factor in me coming here, and Steve Walsh as well.
"I know him from my days at Leicester, so I felt really wanted by everyone at the club and I just can't wait to get going now."
