For me, once you have kept very large, large, medium, small tanks, in various configurations, you want to try something new. This is not for everyone.
In my case, I had a small space that I would normally have put a houseplant to bring a living thing to a bare corner, but I saw a friend's small bowl with thriving plants and a single guppy that looked very nice and really appealed to me esthetically. It is a challenge to keep this tank, but I really enjoy it, it is pretty, and the fish is apparently very happy. I tried another very small fish, actually smaller than the guppy, but the fish was obviously not happy and I changed him for the guppy.
He swims around, explores, and takes food off the tip of my finger. His color has improved since I first got him, so I feel comfortable that he is not suffering.
Finding small species of plants and pruning them, keeping them thriving so they filter the tank and provide oxygen is a challenge that I enjoy.
I hope that helps explain it, at least from my POV.