I think it's pretty simple, you just take the starcraft AI, reduce the number of units it can handle at a time, maybe simplify its economy logic, and then let it go. Easy.
The interactions between individual unit types in a game like DOTA are much more complicated than, say, the interactions between marines, zerglings, and zealots; there are many more unit types, too.
as someone else pointed out, the interactions between the units are like fighting games. Computers already have the advantage at fighting games (like Streetfighter).
Micro and unit control is where the AI has the advantage.
The complexity of the unit interactions also has a strategic layer (drafting, lane assignment, timing) which may be far more difficult to train and isn't just purely the micro aspect of the problem.
