Walmart is dropping VISA as a method of payment because of high fees, Bitcoin provides an easy method for paying just as VISA but with lower fees.  Losing VISA as a payment option could affect many people that rely on VISA to pay for items at Walmart.  Bitcoin is a great alternative to VISA because of the adoption rate.  Currently the Bitcoin market is worth nearly 12 billion dollars US but it's difficult to spend. There are many online stores that accept Bitcoin but few stores that take it over the counter, making Bitcoin an untapped market for many retailers.  Overall, Bitcoin is an untapped market that could easily replace VISA and has lower fees.

This article goes into greater detail about Bitcoin replacing VISA: Walmart Canada and VISA Showdown, Will Bitcoin Emerge as the Winner?