There are a few reasons where there aren’t any viruses for Blackberries.
1) Motive is lacking
If your goal is to write a virus to do the most damage or control the most nodes possible, you probably wouldn’t spend the time researching and writing a virus for the 2nd market share holder. Symbian OS held about 48% of the Smartphone market as of Q4 2008 whereas Blackberry OS held about 20% of the market.
2) Technically difficult
A bigger reason for not writing viruses on the Blackberry is the technical difficulty of it. In order to infect as many nodes as possible, a virus needs to be able to self propagate – sending itself out to other nodes and installing and repeating. As mentioned above, due to the way Blackberries handle email attachments, at this time, it’s not possible to write a self-propagating application.
Another technical issue is that the Blackberry OS is better secured and thus harder to write a virus for over something like a Windows PC.