1. The Sun's Heat: The inner solar system was much hotter than the outer solar system due to the Sun's intense radiation. This heat prevented lighter gases like hydrogen and helium from accumulating near the inner planets. These gases would have simply been blown away by the solar wind.
2. The Sun's Gravity: While the Sun's gravity pulled in material from the protoplanetary disk, it also had a strong enough influence to prevent gas giants from forming close to it.
3. Smaller Size and Mass: The inner planets were much smaller and less massive than the gas giants. This meant they had less gravity to attract and hold onto the lighter gases. The inner planets were more likely to capture heavier elements like rock and metal.
4. Timing: The inner planets formed earlier than the gas giants. By the time the gas giants started forming, the solar wind had already cleared out most of the lighter gases from the inner solar system.
In summary: The combination of the Sun's heat, strong gravity, and the inner planets' smaller size and mass led to the formation of rocky inner planets instead of gas giants. The outer planets, further from the Sun, were able to retain their gas and grow into the gas giants we see today.
