Why is Budai often associated with the future Buddha Maitreya?
Here is a reference to the Japanese Miroku Bodhisatva.

Another reference says that Hotei was Kaishi Zen priest and a Boddhisattva, the future Maitreya:


  Hotei stands out from the other six Lucky Gods because he is the only one known to have been a real person, rather than a mythical being. According to Chiba, his Chinese name was Kaishi. His birth date is unknown, but his date of death was in March of the year 916. He was a Zen priest who could quote Buddhist text verses almost nonstop. Some sources affirm that he was an  incarnation of the Bodhisattva Miroku (Maitreya in Sanskrit), the Buddhist Savior of the Future (Giraud 404). However, some of his actions would not be  considered particularly saintly. For example, he would beg for meat and fish  even though they were foods forbidden to priests.


Bits of details about claims to Maitreya and where references of it could be found in the pali canon.