An attackers motives for attacking a server are entirely different than their motives for attacking a desktop. The former are generally attacked to gain access to the data residing on them, the latter are attacked to gain control over the machine, so that it can be used to generate income for the attacker.
- By re-directing searches, to generate click-through advertising income.
- By utilising "free" disk space to distribute spam and porn.
- By co-opting the computer into a botnet - which can be used in a number of ways to make money for the attacker.
- Etc. Etc. Etc.
The difficulty of course is to get your malware onto your target, and that's usually a good deal harder to do on a server, because servers are generally operated by professionals.
So, for that reason, the majority of malware is written to attack desktops, because their average User is much more likely to load a "malicious" package.
Now if you were a malware writer, and you wanted to get maximum payback for your effort, who would you attack .... Windows, who have over 90% of the desktop market .... or Linux, who have less than 2% of the desktop market ?