Here I always thought it was because it was a smart phone.
Check it out:

Why can’t President Barack Obama use an iPhone?

“I’m not allowed, for security reasons, to have an iPhone,” Obama told a White House audience Wednesday, according to Agence France-Presse. Instead, the news agency said, Obama has to stick to his BlackBerry, a valued brand in Washington due to its strict security controls and strong encryption.

That’s not the entire story, however. BlackBerrys also offer a high level of organizational control and security that few other smartphones can match.

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