Security isn't the limitation. Carriers have their security and most security-minded enterprises will add their own on top of whatever the carrier delivers.
The drivers are reliability and cost.
If you're aware how disruptive it is for Blackberry users when users are unable to send or receive message then its easy to understand why corporations are reluctant to move more applications to wireless when a wired network is available.
And even then, the messages delivered to one's Blackberry are also delivered to one's desktop so the user is not relying solely on their mobile device. This is done for good reason.