These questions always come off like asking 'why does anyone like brunettes?' Because people like different things than you. I love to read books, that's why I read.... books. A small phone is fine for email. I'm not blind. Your experience with the world is not the default one for all humans. You might as well be asking why do people like "blue".
Why do I want a smaller phone? Because I don't wanna carry around a dinner plate in my pocket. I wear jeans, I don't want to also have a fanny pack just to hold a phone while I'm on my motorcycle. I don't like having some giant 'tech bling' smacking against me and dragging down my shorts while I'm riding my bicycle. When I go to a bistro for lunch I don't want my phone to take up half the table if I sit it down. I do things where a large phone is annoying. I don't need or want a large phone since 'the phone' is not a lifestyle for me. I just want the tech to 'do it's thing' and then disappear into the background of my life.