There a a some touring bikes designed with full suspension (front and rear). The difficulty is getting the luggage racks to work with suspension.
Tout Terrain Panamerica
Moulton AM
The best reason to use suspension is if you are riding on trails with a
washboard surface: regular waves that stop you rolling smoothly.
Most tourists find that they can ride shorter sections of trail without suspension so we dont need the weight, expense and complication of suspension.