UX/UI design mainly. All social media follows a fairly similar pattern, post pictures/text and other people react to it. The difference comes from what the UI of different social media platforms emphasizes, Pixelfed emphasizes pictures, Mastadon emphasizes short-form text posts, Lemmy emphasizes variable posting types with extensive comment threads. Because each one emphasizes a slightly different medium, they can design their UI/UX to prioritize their whole schtick while making design decisions that may not be good for other mediums. Everybody has a different preference for the ways they like to consume content, so I understand that people prefer Mastadon/Pixelfed even though I personally prefer Lemmy.