At the risk of sounding sarcastic, it rusted because it got wet. Even if you only used it once in four years. Once the rust started, it picked up enough moisture from the air to continue. Then you didn’t catch it soon enough.
On the other hand, you might well ask why they built a
marine air conditioner with
steel parts. But you’ll probably get the same answer that I got when I asked why the mild
steel frame on my
scuba diving compressor rusted away. I didn’t catch it either. After all, the frame was suitable for a basement in Chicago on a dry day. Or the
aluminum base on my
refrigerator compressor after a "minor" leak in the
cockpit. Or the
exhaust mixing
elbow on my
engine after 300 hours. Or the steel screws holding the "marine
water pump" together. Or my
marine stove which is all SS on the outside, while what’s left of the inside isn’t. Or ....
Manufacturers cut corners, or just don’t really pay attention to details. Stuff corrodes whether you use it or not. Scraping,
sanding and painting are a continual job. I just figure it keeps me from breakfasting in the local bar.