To try to capitalise on the Spiderman name and make money.
To quote from The Huffington Post who covered this:
A fourth installment of the hugely popular [original] Spider-man franchise was
planned, with director Sam Raimi and star Tobey Maguire returning to
their trilogy of films that had earned more than $2.5 billion at the
global box office and generally been hailed as a standard-bearer in
big-screen comic book adaptations.
But by that Monday, Raimi's dissatisfaction with the script and the
producers' eagerness for a new movie had come to a head. In a flash,
the sequel was kaput, and a reboot was ordered up.
...Reboots of film franchises have been typically launched many more
years later than that. But today, "five years is a lifetime in the
movie business," says Sony Pictures co-chairman Amy Pascal. "I wasn't
troubled by it."
..."The only time to take a break is when your franchise fails," says
Avi Arad, a producer of the film and former CEO of Marvel Studios.
"People want Spider-Man, so it's our responsibility to give them
something new, something different and start a whole new generation of
...Either way, the future possibilities for more Spider-Man are again
limitless. Says Arad: "This can be so many movies."
Variety wrote a nice article about it which included the reasons:
Struggling with one of the comicbook industry’s most popular
superheroes was starting to get embarrassing — especially when
“Guardians of the Galaxy,” starring a cast of misfits unknown to most
moviegoers, broke out as a bigger hit in 2014 than the last
“Spider-Man” movie when it made $774 million. “The Amazing Spider Man
2” earned nearly $709 million worldwide.
“The Amazing Spider-Man 2” just wasn’t the billion-dollar hit that
former Sony Pictures co-chair Amy Pascal had been looking for last
year — and needed to be in order to keep full control over the
character. That movie became the lowest grossing film of Sony’s five
Spidey films to date, and forced the studio to instantly start
rethinking its plans for the character, even giving up a piece of the
The Sony chairman simply commented:
“This is the right decision for the franchise, for our business, for
Marvel, and for the fans,” said Sony chairman Michael Lynton.
It's pretty clear this has been all about making money. Sony want to capitalise on the Spiderman franchise and have been frustrated at their attempts to do so thus far. When the first reboot came out, plenty of actors from the first trilogy were pretty scathing about.
Of course, that doesn't mean Sony haven't made money. Far from it. On a $1.078 billion total budget across all five films, they've made $3.963 billion worldwide box office gross. All of the films have been box office successes world-wide (although the first reboot performed very poorly in the US). But that hasn't been enough for Sony who have demanded ever more success from the franchise (hence the second reboot).