I feel like Windows gets targeted the most because there are just so many computers running it. If 80% of computers out there ran OSX I wonder if anyone would give a shit about the next vulnerability in Windows.
However, speaking about Windows, historically has been pretty insecure by default. Microsoft started giving a shit only recently, after the failure that was Windows Vista and the bad press associated with it.
And btw, I don't know what you mean by the early XP era, but you're probably referring to a timespan of at least 20 years.
as for Vista ... http://en.wikipedia.org/wiki/Security_and_safety_features_ne...
but the real point is that it's just nonsensical to say that Microsoft has been more focused on security as a reaction to bad press of Vista - whether you like or hate Microsoft or Windows or Vista, there's just no interpretation of the timeline under which that makes any sense.
You could also argue that the area of attack becomes smaller and that in general this is a good thing. However, personally I'm not concerned with script kiddies that are just trying to have fun on other people's expense, as based on my own usage patterns I could never be infected, because such people do not have the capability of distributing such mallware to me (as I'm only getting software from reliable sources). The threats I'm worried about come from the people with in-house expertise and that are earning money from this activity, as they do have the means for mass distribution and the potential for real damage to your bank account and online identity.