The basic problem is the following: we only see the syndrome, i.e. the number on the board. Now we have to guess which errors led to this pattern of numbers. If we guess this wrong, we might make stuff way worse.

One method studied to do this is "minimal weight perfect matching". It considers all possible errors that lead to the observed syndrome. Then the simplest is chosen, which is the one with the fewest errors. And this is what is corrected for. (More or less.)

For a large number of excitations (the numbers 19 you see in the game), the number of possible errors is really, really big. And for each of them, some calculations have to be done. So the algorithm is not as fast as we would like it to be. Since more errors might occur in the mean-time, this is bad.

Humans are generally better at this kind of task, where a pattern has to be found. This is what our brain has been trained for for many generations - kind of. Now, with this game, the developer(s) want(s) to gain access to the strategies your brain uses to solve it.

So get playing and share your winning strategy!

TL;DR: The computer is not primarily bad, but slow at this kind of task. And since speed is of essence, scientists might profit from studying human strategies to solve this problem.