From identifying the causes, i believe we will be able to get solutions.
I'll make a list, not only for passwords, instead for losing access to various means of storing the coins.
1. Carelessness, especially from the days the coins worth much less.
2. Overly complicated storing methods.
3. Over-confidence on their capabilities (eg. thinking that they can keep in mind a complicated password for 10s of years).
4. Accidents (from spilling coffee onto the paper wallet to car crash or stroke and brain injury).
5. Storing the wallets on exclusive IT related supports (HDD/USB/whatever) and those getting destroyed or declared obsolete and thrown away.
6. Storing the wallet info in only one place.
7. Lack of understanding crypto, computers, web/services.
The order is.. what came into my mind.
I don't see a proper solution for them. Using/keeping only seed should be basically it, but if one keeps it "in clear", then he won't forget it, but others may understand what it is and steal the coins.