As to why, the convenience of never missing a call and the fact that some people are rarely home are two reasons. Wireless-only consumers who buy a lot of minutes and get free long distance can pay about the same for their phone service as they would for the usual combination of local phone and long distance charges. But there are drawbacks to going completely wireless. Receiving signals is difficult inside some buildings. Multiple phone extensions aren't possible with wireless phones, which can also be lost or stolen.
Consumers have network reliability and battery life worries, too.
But the biggest argument for a wired line is that people need one to connect a fax machine and for Internet access. Wireless still wasn't that large a competitor when the Telecommunications Act of 1996 became law. But today it is an acknowledged competitor to landlines. With many different companies offering local and long distance, wireless has changed the dynamics of the industry. It's an integral part of a company's business plan.