You basically got to have your own little reliable niche
I think one big problem is originally. So many indie games are essentially clones of the games that the developers happened to like. Zelda-likes, rogue-likes, greyscale puzzle platformers about depression. There are literally hundreds of examples of the first two of these, and not as many but still weirdly a lot of the third. But without something to make it stand out, casual players will come across the game and think “This looks neat, but basically the same as about 4 other games in my wishlist that are already very well reviewed. Maybe if it starts getting rave reviews, I’ll add it to the queue”.
Not to paint all indie games with one broad stroke, the most novel game ideas out there are also usually from indie studios. I don’t have numbers and I don’t know about longevity, but I bet that games with novel ideas get more initial downloads