What is the reason behind the multiple Spider-Man reboots? [duplicate]

To try to capitalise on the Spiderman name and make money.

First reboot:

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 Spider-Man lovers."

...Either way, the future possibilities for more Spider-Man are again limitless. Says Arad: "This can be so many movies."

Second reboot:

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 franchise.

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.

Conclusion:

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).