NYMag explains one of the reasons:
Webb, whose (500) Days scored a Golden Globe nomination and whose name seems to have predestined him for this job, has long been a favorite of Columbia chairman Amy Pascal. We're told that last year she very nearly hired him to direct the adaptation of Michael Lewis’s Moneyball after she pulled the plug on Steven Soderbergh three days before shooting. She ultimately chose Bennett Miller (Capote), thinking that Moneyball might be hurt by Webb's whimsical style, but she views him as a latter-day Cameron Crowe for the economically and socially angsty Generation Y, and thinks he'd be perfect to explore the conflict within Parker.
This ends up being a huge savings for Sony, since Webb will helm for $10 million, compared to the deal Sony had with Raimi and Tobey Maguire, who were set to get more than 25% of the gross profits - a deal pretty-much unheard of in Hollywood.
NYMag explains that with DVD sales plummeting and an expensive director and cast, there was a real chance that this latest Spider-Man flick wouldn't turn a profit at all for the studio. With Webb signing on for pennies (in Hollywood terms) it gives Sony a little more breathing room.
And last we heard, Webb is also signed with Universal to do a remake of "Jesus Christ Superstar" which tells me that we are going to be hearing a lot more about this guy in the near future.