1.It was a passing fad/In it for all the wrong reasonsI think most of the time I have seen someone sell their entire collection was because they were in it for the wrong reasons. Mainly for attention, showboating or to make money flipping. Once one of those 3 avenues dry up which they certainly will. Some people give up. I used to know this guy called Jay Bandz. He used to spend thousands on only the rarest games. Weather they sucked or not. His entire collection consisted of only the highest end games. And you could tell he was never in it for the love of it because days later he'd be trying to flip the game for more than he paid. Once the insta likes wore off and all the "OMG, I'm drooling" replies stopped coming. The game became worthless to him. He was hardly a gamer at all. Hardly played. Hardly cared. In it for all the wrong reasons.
Others just lose interest because they have ADD and just switch fads constantly like me lol. But I would never sell my whole collection.
Financial needs or a babyI know you said excluding this one, but figured i'd include it. Usually people sell their games because they have a baby. Which I kinda don't get in a way. Sure you need money but your baby should have already had the means of being raised based on your income not your hobbies. I think giant changes are made when a baby is born. I think bonding with the games is a better idea
To buy a houseI forget the user on here, but I remember someone selling a bulk of their collection to buy a house. I see this very often. It's something I have pondered but my collection is not nearly large enough. I think sometimes you value what is more important in your life. And having a house to pass down to your kids, and having something you can call home wins. I definitely get this decision. I think video games can be repurchased slowly. But a house is forever.
DepressionProbably the saddest one, sometimes people lose interest due to depression. It can make you hate things you once loved. And eventually you never get that spark back. It's crippling. Anxiety especially can convince you stuff is happening that's not. It can be hard to shake that cycle.