They are big, and bulky. They don't fit into a backpack. And you can't simply hold them in one hand and read, anywhere you sit. Like a bench in a garden. You need a space dedicated to reading them.

In terms of utility, hardcovers are strictly worse.

Sure - if I have a nice chair to sit on in the evening, with good lighting, and can rest it on something, they're quite nice. But the bulk of my reading is either outside of home (so they need to fit in my bag nicely) in an uncomfortable chair, or while lying down so I need to hold the book up. Weight/size really matters here.

When I was a kid I asked around, and in those days the reasons I was given were:

1. They're durable - less likely to fall apart - and may last generations.

2. The font is bigger.