The reality that most mainstream techs usually forget is that Windows and programs like IE, MS Office, SQL server and so on are targeted in the first place for one reason - their ubiquity. The fact that some of these platforms are inherently open to abuse is a bonus.
Let me give you an example to clarify the point. If you're a cracker and you want in to a system for nefarious gain will you:
a) Focus your efforts on breaking a system you know is on millions of computers that will have the data you want
b) Focus your efforts on breaking a system that you know is on 100s of thousands of computers that will have the data you want
c) Focus your efforts on breaking a system that you know is on thousands of computers that will have the data you want
If you answered a) and you found that those platforms to also be easy to beak into then you'd be a happy little cracker indeed.
Mac OS was never a high profile target. Not when oppertunist hackers and those after a large portion of computers could simply focus on Windows. As numbers of Mac users with valuable data rose, so too did the interest that malicious little code monkeys had in fruit rather than glass, if you get where I'm going. Same happened with Linux as more home users adopted it (and failed to secure it properly) and more devs wrote stuff for it (and introduced vulnerabilities).
I still laugh to myself when remembering what a Mac-toting marketeer once said to me, "I don't need security. Macs don't get viruses. That's a windows thing. Macs are just better"
Er....guess again, tango brain. Back to your fisher price, if you please
So, yeah, I think you're right - a large portion as to why Macs are no longer considered super secure is because the focus is now on them in the hacking community because they are becoming a big enough target to bother with.
The only secure computer is one not connected to anything. In the age of the internet, who wants that?