The reason is because of pressure. You can go on a THOUGHT EXPERIMENT diving deeper and deeper into the “atmosphere” of Jupiter. Start at the top where the mixture of H₂ and He (diatomic hydrogen gas and Helium) are very low density. As you travel deeper because there is more and more H₂ and He above you, the PRESSURE increases. Now, as the pressure increases t,he density of the gas mixture increases when the pressure exceeds a certain value (that is when you are deep enough), the gas mixture converts to a liquid, .this is a pressure effect. Finally at still higher pressure the liquid mixture becomes solid!!!

When pressure increases on any material the atoms that make up the stuff get closer and closer together. This “getting closer” means that the density increases. The density of any material is DEFINED as the mass of the stuff per unit volume. So, the density of water vapor is rather low like 1 kg per cubic meter… as the gas compresses it eventually becomes a liquid water with density 1000 kg cubic meter. I hope this helps.

In Jupiter, the hydrogen in the atmosphere is in thegas phase. As you go towards the center of Jupiter, the hydrogen undergoes a phase transition to the liquid phase.

Why does this happen?

Hydrogen, and materials in general, will change between solid, liquid, and gas phases with temperature and pressure. In the case of descending toward the center of Jupiter, it is the increase in pressure which causes the transition. The pressure on some area within Jupiter's atmosphere is the weight of the atmosphere above it (attracted toward the center of Jupiter by gravity - the same thing happens on Earth, and anywhere else with an atmosphere).

In the outer atmosphere, there is very little that is higher and pressing down on the gas, so the pressure is low and the gas molecules are able to move around without interacting. As the pressure is increased, the molecules are forced closer together and start to interact with each other. Eventually it is more favorable for them to interact and stick close together rather than be spread out, and then we call it a liquid.

Phase transitions can happen because of changes in temperature, such as when water freezes or boils. But phase changes can also happen because of changes in pressure. Water, for example, boils at lower temperatures up in the mountains, but it boils at higher temperatures near sea level. Further away from Jupiter’s center there are lower gas pressures and temperatures, low enough that any liquid hydrogen boils. Further towards Jupiter’s center the pressure and temperature increase enough that liquid hydrogen can exist; it is stable there.

The pressure inside Jupiter is so high, that it "compresses" the gas hydrogen into a liquid hydrogen. When the pressure increases, the density of states of matter also increases, and the liquid hydrogen has a much higher density than gas. In fact, the same phenomenon happens to steam and water. Increasing pressure on steam also turns it into water.