Beckham, whose LA Galaxy team are between seasons, had hoped to win his 100th cap when England face Switzerland at Wembley on February 6 but was not picked for the squad.
"I am not surprised," Ferguson told reporters. "I think Fabio is correct in what he said that he is not match fit. That is the reason he's not picked. I think that's straightforward.
"And you have to think about Fabio's position. He has to win that game," Ferguson added.
"It's his first game as manager of England. He is a foreign coach who has come with a great CV and reputation and so the Switzerland game is an important game for him.
"He has laid down his marker quite firmly. He wants players who are match fit and of course David has not played any football."
Beckham, 32, won 60 of his caps during his time as a United player.
Ferguson's own attention is focused on the race for the Premier League title with United looking to stay ahead of Arsenal and Chelsea by winning at Tottenham on Saturday.
United beat Tottenham in the FA Cup last weekend but Ferguson said there had been a "wind of change" at White Hart Lane since Juande Ramos took charge at the end of October and lifted them towards mid-table and a place in the final of the League Cup.
"I have read about his discipline in terms of the regime of diet and the intake of alcohol and that type of thing," Ferguson said.
"These are things I had to broach myself when I came down here and he's right. He's 100 percent correct. His record at Seville stands testimony. There's no doubt about that."
United, who have won nine of their last 10 games to lead the title race on goal difference, have no fresh injury worries but defender Gary Neville, who has been out since March with leg injuries, and French striker Louis Saha (knee) are not yet fit to return.
(Editing by Clare Fallon)