This post is going to discuss the decision to use Bluetooth HID over other communications.
1. User Experience
Bluetooth interface saves the user from switching cables. Wired communication method would use the same USB port as the charging cable. Using power charging cables is considered to be poor user experience.
Wired HID cannot be adopted on all phone types. Since not all smartphones have standard placement of USB socket, the keyboard should be able to adapt to the phone type. Wired communication also obstructs user's interaction with the smartphone/keypad.
Wired HID would require separate USB type for each phone type. Having multiple USB types for the same device will increase manufacturing complexity and cost of capital.
These are 3 major influencing factors to select wireless communication.