Because most airlines have become monopolies at their respective hubs so their loss would severely inconvenience a ton of people so government is encouraged to prop them up.
For example, if Delta went under, Atlanta, Detroit and Salt Lake City would lose a total of 50%+ of their flights. That would be absolutely devastating.
Devastating to individuals sure. I hold a pretty high bar when it comes to something being truly critical enough for the government to bailout, economic concerns never meet that bar for me.
If we allowed markets to become monopolized we have to deal with that when the bill comes due rather than kick the can down the road.
If Delta went under another carrier would buy their planes and gate access at those airports, it might be chaotic for a short time but if there is enough demand to fly from point A to point B someone will provide the flights.
If Delta went under, one or more airlines would rapidly go "hey, there's a proven demand that's suddenly unmet", and there would very quickly be replacement flights.