Decks receive a lot more wear and tear due to foot traffic, shovels and moving chairs and tables. Also because it’s a flat surface, moisture can accumulate and sit on top of the surface.
Moisture can also enter through other areas that we aren’t able to stain, such as underneath the deck or lattice or other pillars that touch the ground. Plus because it’s a “stain” and not a paint means it’s thinner. This is especially seen with clear coats and semi-transparent stains, so the sun breaks it down faster and then the wood can grey.