Re: Which begs the question
You haven't specified a subject but I'm going to assume that you are talking about GNU/Linux. There are two answers to your question (neither mutually exclusive). The first is that you're wrong - there actually aren't a "vanishingly small number of attempts to exploit them". Companies face active attempts to compromise their GNU/Linux systems daily. It is end users who don't see many attacks.
And that last part leads into the second answer which is regarding the disparity between attacks on GNU/Linux end users and those on Windows end users. The reasons are fairly elementary. If it takes the same amount of effort to craft an attack on either OS, are you going to direct your malware efforts at the OS that has a huge proportion of the total end users, or the one that has a small proportion. Furthermore, are you going to target the userbase that is a mix of technically competent and technically incompetent people, or the one that is stripped of the technically incompetent people?
Short version: For back-end systems, your question is actually wrong - both GNU/Linux servers and Windows servers are actively targeted because they have equal value. For end users, the reason for the huge disparity is that the two sections do not have equal value.