Another reason why IE is more vigorously targeted: it is patched the least. Some systems deliberately run downrev versions of IE for various reasons (they are pirated and thus afraid of Windows Update, or they are based on a frozen IT build of Windows which only gets updated quarterly, or whatever).

So, even if Microsoft releases a patch for this latest IE vulnerability today, millions of PCs will remain unpatched for months. Valuable targets -- members of the next botnet.

By comparison, Chrome is much more aggressive about updating itself. For example, consider Chrome 6.0's short lifespan:

http://www.netmarketshare.com/browser-market-share.aspx?qpri...

But all major browsers are not targeted with the same vigor. IE is targeted much more vigorously because the criminals stand to gain much more from their efforts than if they target, say, Opera.

Also, Chrome's sandboxing is designed to assume the browser will have exploitable vulnerabilities, so there are two hoops that the exploit must jump through instead of just one. In this latest IE vulnerability there is only the one hoop and then on most machines the exploit has acquired administrator privileges.