I'm trying to come up with a simple and satisfying explanation, but the best I can do is "Starcraft is a complex game of balancing your ability to attack and defend. AIs have difficulty with situational awareness."
But I think that case analysis of what I've observed may be more telling.
Case 1, The AIs tend to over-react, or under-react. For example, when a Zerg player sees flying units they may start to go scourge heavy. If a human player notices this, they may build a Wraith or two, causing the Zerg player to waste a lot of money. This can happen naturally between AIs as well. Terrans depleting the Command Center's invisibility sweep too soon is another one... Something is getting hurt, but you may want to wait until after the Tanks are in siege mode before attacking (not while they are converting).
Case 2, Lack of Memory. This one happens a lot. You'll see an AI do something bad. Then, in a situation where circumstances obviously wouldn't have improved, try again a few seconds later. I'd want to blame the fog of war, but futile attacks on static towers is a common example.
Case 3, Fight or Flight. Sometimes you'll see units fleeing from a battle they cannot win. Sounds good. But sometimes they are being pursued by units that can pick them off during retreat. And they aren't fleeing for more support, they are just avoiding a bad situation. When in reality, those units are dead no matter what, might as well try to take down an enemy unit or two. The inverse can happen too, where units stand and fight in a situation they could run away from and get reinforcements. I've heard predicting combat outcomes is really difficult in SC.
Case 4, Under utilization. Vultures are the unit that stands out in my mind as one that an AI can handle better than a human. They are fast, have a great punch, and can deploy mines really effectively (some bots mine much of the map). What was a harassment unit becomes an offensive unit that can hold its own in a "fire/flee/repeat" pattern (imagine having 5 of them do that in the same area). High Templar are the opposite. I tend to expect AIs to do poorly when storming, it is currently a highlight/joy to see them utilize the ability effectively. But the ability is meant to discourage enemies from grouping units too closely together. Which allows for Carriers and Mutas to do a lot more coupling / damage than they would be able to otherwise do. Under current circumstances, it would be possible to see a dozen or more Mutas being crippled in a single storm. I rarely see nukes/Ghosts or Defilers used. Queens also seem to be under utilized.
Case 5, Target Prioritization. When you see a Carrier, kill it rather than the Interceptors. When you see a Medic or two, kill them before the Marines. Same with SCVs repairing in some cases (with sufficient firepower, the unit will be dead faster than it can be healed). One AI loves Carriers, and part of the reason their strategy works is that a lot of units go after Interceptors rather than Carriers, allowing the Carriers to retreat (the AI judges when to do this well) and rebuild.
It isn't that AIs can't play at a professional level, but this represents the current level of the bots. Looking over it... The AIs can pull off things we can't, but professional level situational awareness / judgement is tough.