Why is it very difficult for a hot air balloon to attain high altitudes?

Hot air balloons struggle to reach high altitudes primarily due to the decrease in air density as elevation increases. As the balloon ascends, the surrounding air becomes thinner, reducing the amount of lift generated by the hot air inside the balloon. Additionally, the temperature difference needed to maintain lift becomes harder to achieve, as the burner must work harder to heat the air to a sufficient temperature. Finally, safety concerns and equipment limitations also restrict the maximum altitude a hot air balloon can safely reach.