IE is on pretty much every Windows box. The less tech savvy users have a tendancy to use it because it's there. These are the people that are easy to target.
If Chrome, FireFox or Opera had been the default browser on Windows, the situation would be different, I'll bet.
Of course, the way IE works also plays a part but for me we all too often miss the point of why M$ products are exploited most in the first place.
And no, I won't be drawn into an 'oppertunism vs mass targets' debate.
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