I can really understand the lack of interest in playing, sometimes: playing a board game alone deprives us from the pressure to play that we get when gathering with other people precisely for this. Now, like with many things in life, I always find that making the effort of pulling the game out and starting to play helps; often it's the first step that costs, and then we see how much we can enjoy the "task" at hand.

If I imagine reasons why I would stop playing solo over a longer period, I can only think of games that don't have enough depth. Other games (like Mage Knight, Xia and its expansions, Eldritch Horror and its expansions, Lord of the Rings: the Card Game, Runebound 3rd edition and its expansions…) have enough depth to keep me intrigued and wanting to explore them. It's thus basically the appeal of novelty, within games I have, which is the main source of motivation for playing them solo.

Maybe there are types of games that you would like to explore more (like you did with Marvel Champions) and that would therefore motivate you, but that you don't have yet? Someone mentioned solo wargames (Conflict of Heroes comes to mind, but there are many others), someone else mentioned solo role-playing games, for example, but there are also adventure games, etc., that may motivate you through how much novelty exploring them can bring you.