Two reasons that I can see.
First, there is the difference described in general relativity. General relativity theory places gravity in a position much different from the other fundamental forces. Gravity satisfies the law of equivalence. The other fundamental forces do not satisfy the law of equivalence.
No one understands why gravity is different from electromagnetism, strong nuclear force, and weak nuclear force. Einstein himself was uncomfortable with this difference. He made an unsuccessful attempt to explain this difference which is sometimes called 'unified field theory'. So there is an self confessed shortfall in Einstein's understanding.
Second, gravity works over a far greater distance than the other fundamental charges. This is because gravity has only one type of charge, which is positive mass. There can't be any negative mass to as can be proven with the law of equivalence. Electromagnetism has two types of charge, which are positive and negative. When the two charges are close together, then the two charges cancel each other out at large distances. So gravitational forces beat electromagnetic forces over very large distances. However, mythological stories seem to contradict this.
Velokovsky's historical theories requires short range forces that dominate the dynamics on a solar system size scale. His comets in 'collision theory' requires all significant forces to be short range. If gravity was significant, then his narrative can not possibly be true. For instance:
Velockosvky claims that Saturn was once close enough to the earth for humans to see the rings. If gravity really existed, then Saturn's gravity would have destroyed the earth at that range.
Velikovsy claims that Venus was once a comet that came close enough to the earth to be seen. Venus is made mostly of dense rock. So if there was gravity, Venus would have destroyed the earth.
Velikovsky claims that a piece of Venus tore lose from Jupiter. If there was gravity as understood in Principia or GR, then gravity would have pulled Venus back into Jupiter.
Velikovsky claims that Jupiter came so close to the earth, that animals from Jupiter were pulled to the surface of the earth by electromagnetic forces. These he claims were the locusts and other animals that appear in Exodus during the Ten Plagues. This also explains the 'lightening' seen as fiery rain. However, this requires forces that were restricted to the surface of Jupiter. Gravity would have pulled more material from Jupiter to the earth.
Velikovsky justifies his historical narrative with the hypothesis that oral narratives are more reliable than written records. He is also claiming that only he has the insight necessary to understand oral narratives. So if an oral narrative as he interprets it contradicts Principia OR GR, then Principia or GR must be wrong. So if the oral narrative indicates that all forces, as Velikovsky understands them, are short range, then there can be no long range forces. Hence, no gravity.
Goodbye, Gravity! We never knew ye!
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