Probably because a large hot gasious explosion is a lot worse in a confined space.
Throw a full airosol on a bonfire and watch it go pop and have a laugh.
Put that same airosol in a house let it go pop, and see how much of your house is left afterwards. Theres also the prospect of structural damage to the support of the underground structure which would make it very hazardouse for any emergency services that had to go in and rescue any potential victims, who would most probably have servere percusive and burn injuries.
And if there was a propane propelled car a few spaces away which didn’t also go pop, but had a leak, youre back in asphyxiation territory as stated above.
https://www.youtube.com/watch?v=KAyuq8sDCeg
OOOOOOoooo….. ouch!!!!..... and thats with the roof off the building!