I'm speculating as to the business reason why, but I can tell you EmailScan was an additional layer of security and as such was redundant. Also a common user scenario exposed problems with it that we had no power to solve which meant issues being reported against the feature where all we/Support could advise was to disable it, or remove it.
The redundancy comes in two places -
1. If the Exchange server is protected (which it should be), your mail was scanned twice; by the exchange and by the client.
2. The on access scanner of the client, because Outlook writes attachments to a local temp folder in order to open them.
The common complaints against EmailScan were -
1. If the client User was a roaming user, e.g. connects via VPN, they would incur performance hits whenever receiving new mail
2. Even Users who were not roaming but if the network was congested, they would incur lag or notice an unresponsive Outlook client (while scanning was occurring)
I don't know the real business reason but I can see reasons why.
Feature might be back in the future: