Well basically as mentioned, the user and permission system of Linux means vital OS files are usually owned by the root account and access is denied to other users. This means that if someone randomly executes a hypothetical
Linux virus they get emailed, which would assume the current user's permissions, it could not affect vital OS files.
Also the fact that there are so many different things you could or could not have installed on you Linux pc means it's harder to target the OS, or to even write working code.
There are a lot more complex things (exploits to elevate code's permissions to those of root's, virii writers not 'crapping in their own back yard', etc) but that's a start.