It is an un-answered question. There are only theories.
All allergies, including asthma, have increased dramatically and continue to do so.

It is the immune system treating a harmless protein as an invader.
The plethora of chemicals that we are all exposed to in the modern environment has been suggested.
Over-zealous hygiene has also been implicated.
The theory goes that if the imune system does not encounter any pathogens, it may just lock on to anything that turns up.