Let aside the personal preferences, 60% was popularized by the gaming crowd:

1. Small means the mouse is closer to the keyboard 2. Games often rely on wasd instead of arrows 3. 40% cheaper for alternative switches and keycaps, or higher margins for the producers 4. Style and look may feel like more exclusive, competitive, make hands look bigger. Fashion is important in this community.

Thanks to their firmware, they are very usable but it is really a matter of personal preferences.