Fabric life often depends on how well the builder applied the
UV blocker, which is most often a layer of dope with aluminum powder in
it. I've been inside airplanes that had lots of light coming through,
indicating very poor coverage and shortening the fabric's life.
Moisture, even just condensation, corrodes things. Mice and
birds get in and do their thing and cause even worse corrosion. Wind
bangs it around, especially control surfaces. And worst of all, people
mess with your airplane and damage it. They don't even have to have bad
intentions. A cheap, lockable hangar is well worth its cost.

Dan