As a major competitor Amazon does put the squeeze on brick and mortar retailers, but the b&m retailers are also to blame.
Going out to the store is often more hassle than it's worth. The major difference between internet and b&m shopping is service. Online there is no service, but many major retailers offer shoddy service at best. Take Best Buy's GeekSquad, for instance. It is common knowledge that these guys don't know what they're doing and many consumers report them behaving arrogantly. Retailers just don't put their employees up to high standards anymore. Hardly anyone does, really. Employment has become a revolving door with high turn around. Employers pay their employees cheaply, treat them poorly and offer little incentive to do well because they know that as soon as one quits or is fired another is lining up to take their place. This translates into poor customer service.
With online retailers the only customer service consumers risk dealing with is if there is something wrong with their order. Better to roll the dice and hope everything goes well online than put up with the guarantee of having to deal with cashiers, store clerks, returns, long lines, congested parking, and other shoppers at retail.
Also, until recently online retailers selling in the United States were not subjected to sales tax which gave them a huge advantage. All customers had to pay was shipping. But the recent internet sales tax has leveled the playing field and shipping prices are astronomical now.