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I would say a number of factors, OP. Obviously looks. Big, expressive eyes, a charming hairstyle and smile never hurts on anyone. There's also personality, etc. It also depends what "type" of cute you're talking about. The cute like a puppy playing with a ball, highly similar to watching chibis do random things and seeing them as cute? The cute as in a girl being feminine/shy-like? Or the cute as in a girl with aesthetic features such as stated above, bust/body size, etc?
In TL;DR, it would be looks and personality for the msot part.