For two main reasons:

(1)    In simple terms because you have aligned with him.
You means your product or service & him means his needs as customer.

(2)   Furthermore because you have achieved this alignment in an effective way.

Effective way here means that the customer does not overpay to receive the desirable quality while your product is cost effective to leave you profit.

Desirable quality means that he receives a product with attributes he very much wants to obtain while cost effective is achieved by your having lower costs.

Attributes he desires for the product are achieved by innovative ways while lower costs by efficient ways of doing things.

So why should a customer choose you?
Simply because you are innovative and efficient in bringing your innovation to market.

Panikos Sardos