Fear of flying?

uncomfortable beds

Leaving their pets.

The ever-present police state!

Family

Contracting an illness from being on a plane.

Cost (or budget)

Lack of time.

They don't enjoy travel.

They prefer to stay home.

Health reasons.

Rusty

airports

It's just too unpleasant........the aisles on plane too narrow, the leg room is non existent and the seats on planes are too narrow.

Having to sleep in unfamiliar beds.

Fear of bed bugs

Getting poisoned from something in the hotel room mini fridge.

Having their garden get overrun by weeds.

Body searches in airports when carrying dive gear.....always picked for this.

Food allergies or health conditions?

They don't want to leave their pet(s)

They don't want to travel with their children

It's getting too expensive

Their health condition(s) make travel difficult

Afraid to go solo?

germs

I agree with cost.