Because the bots were designed for Battle.net, the protocol spec on 1.0 and 2.0 are completely different. I have yet to see anyone crack the spec for 2.0, and it's been out for a year now. And thanks, ;).
Not much profit in spambotting an RTS, an item based game like Diablo 3 however...
At the moment you just can't make enough money by cracking 2.0 security features to justify the cost and time investment. Once D3 is released people will be able to start selling items for large sums of money, which will give spammers much more incentive to build effective bots.
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