Mostly because the only real exposure to electricity the people who came up with the “classical” elements had was lightning. I’m sure they experienced static electricity from time to time, but they probably didn’t associate it with lightning.

From their perspective, lightning is a very brief moment of extreme light, then maybe a fire if they can actually see the impact site. So it seems a lot more closely related to fire than we would suppose.

maybe there is a scientific explanation of why it isn’t

Nothing about the “classical elements” is rooted in what we would understand as science today. It was just people making guesses about how the world around them worked without the rigor of the scientific process. It was little more than wild speculation.