Alonso has been repeatedly linked with a move away in recent months, and his former boss has confirmed that the Spaniard believes he stands the best chance of challenging for the world title at another team.
Di Montezemolo told RAI: "Alonso is leaving for two reasons - One, he wants another environment. Two, because he is an age when he cannot wait to win again.
"He was disappointed that he has not won over these last years and wanted new stimulus."
Alonso has two years remaining on his current deal, but reportedly agreed to terminate his contract after Ferrari secured the services of Sebastian Vettel for next season.