@Popularity and security
MS Windows is attacked because it is so vulnerable. From failure to install an access security model, to forcing users to run as administrator, to not comprehending basic safe practices in privilege elevation, to allowing code execution from data regions of programs, to allowing any old piece of any old file to be executable, MS Windows is inherently unsafe.
The *nix systems come from a realm whereby it is assumed that they are under attack from outside and inside, and build in security accordingly.
It is certainly possible to do a lot of silly and stupid things to open up the Unix like systems to be vulnerable, but it has to be worked at.