Why Reboot Spider-Man?

From Deadline - 'Spider-Man 4'? scrapped: (January 2010)

[...] Sony Pictures decided today to reboot the Spider-Man franchise after franchise director Sam Raimi pulled out of Spider-Man 4 because he felt he couldn’t make its summer release date and keep the film’s creative integrity.

[...] My sources tell me that Raimi told Sony Pictures: “I can’t make your date. I can’t go forward creatively.” And, so, once he said “That’s it”, Sony Pictures co-chairman Amy Pascal and Columbia Pictures’ Matt Tolmach decided they didn’t want to replace him and instead chose to reboot the franchise.

[...] The events that led to today’s shocking decision to scrap Spider-Man 4 can be traced to mid-December when I saw a December 11th email alerting the pic’s special effects crew that the fourquel would not be starting as planned ”but Sam Raimi has story issues [that] need to be resolved before we are ready to shoot”.

At that point, it wasn’t well known that the Spider-Man franchise director helming the 4th installment had huge problems with the script that has run through screenwriters Jamie Vanderbilt, David Lindsay-Abaire, and Gary Ross. I was told Sam Raimi had been very vocal inside Sony that he “hated” it.

(click the link to read the Sony's official press release)

The reason behind The Hulk's reboot is the negative reviews of the film (It's a different fact that some of people like me liked the old Hulk movie more). And the new Hulk film is also made up his entry in Marvel cinematic universe.

For case of Spider-man Sony and Rami's Clashes are the reason for cancelling Spider-man 4 and then Sony decided for a reboot to save the lose of copyrights of spider-man films. See here for details.