Why did people stop using hydrogen in hot air balloons?

People stopped using hydrogen in hot air balloons primarily due to safety concerns, as hydrogen is highly flammable and poses a significant explosion risk. The notorious Hindenburg disaster in 1937 highlighted these dangers, leading to a shift towards using helium, which is non-flammable and safer. Additionally, helium became more readily available and was preferred for its stability and ease of use in ballooning.