Why can Enel be hit without Haki? In the Skypia Arc Enel is directly hit, although he has a Logia type Devil Fruit. Why can Enel be hit without using Haki? I know Haki was introduced a long time after the Skypia Arc. But other Logia users, like Smoker, could not get hit from Ruffy at that time.

What makes Enel different from other Logia users?
Haki was unknown at that time so any Logia users we saw hurt were due to their specific fruit's weaknesses:


Crocodile has the Sand Sand Fruit which is weak to moisture.  If you get sand wet it clumps and, therefore, can be hit.  Luffy takes advantage of this by getting himself wet, swallowing large amounts of water and regurgitating it, and using his own blood to wet his fists.
Ace's fruit allows his body to be made of fire. Nevertheless, he is burned himself by the Lava Lava Fruit which is hotter.  Lava outclasses Fire.
Enel is simpler.  Rubber is a really good insulator and is, therefore, "immune" to electricity in this story. As electricity is outclassed by the rubber, Luffy can hit him.