Mom and pop stores are gone because they are inefficient (exception: certain niche markets). What rational person would want to pay significant markup and have less to choose from simply to keep an outmoded and inefficient business model going? Personally, I like the idea I can travel anywhere in the US (and many parts of the world) and still pick up a Big Mac when I want one.
About 8 years ago, I arrived in Perth, Western Australia after the long trip from Los Angeles. I was hungry and needed to stretch my legs so I found a MacDonald's about 2 blocks from the hotel. The Big Mac tasted entirely unlike anything I'd ever had stateside. It really sucked, but the fries were good!
Not to mention that many Mom & Pop stores pleased their customers, had a lot of babies and became chains themselves. It's not like Sam Walton started out with $500 billion and 1,000 stores.