The easiest answer that we see in the film is that there wasn't time. When Paul figures things out, Percy (Doug Hutchison) has given the order, Jack (Bill McKinney) has already thrown the switch, and it's much too late to stop things and rewire Del so that he'd suffer much less. The scene is depicted in real time, but doesn't actually happen that way. There are two things going on at once: (1) Percy's management of the entire execution and his orders being followed ("Roll on two!" the order to throw the switch), and (2) Paul's reaction to the revelation about the sponge, the consequences that will inevitably result, and the reaction of the witnesses gathered in the room. It's all done very well to give the viewer a sense of the sequence of events involved, however, it's not actually "real time" as we know it. So, by the time we see everyone's actions and Paul's reaction, it's too late.
When the current is on, Paul could have theoretically ordered to stop the electricity, and soak the sponge before resuming. Given the benefit of hindsight, that would certainly have been the best option. However, it probably would have looked extremely bad if Paul had interrupted the procedure, making it seem like he and the other guards can't conduct a proper execution. It's also safe to assume that nobody has witnessed an execution where the sponge was dry, so nobody had any realistic idea of how long Del's suffering would continue. Paul may have thought that Del wouldn't have lasted much longer anyway, or at the very least would pass out from the shock pretty quickly. Interrupting the execution could have unnecessarily prolonged his suffering. However, to the guards' horror, there almost seems to be no end to Del's protracted ordeal. Needless to say: had the guards known that Del would have to suffer such an extended agony, they would undoubtedly have interrupted the procedure. Unfortunately, they didn't.