Here's my theory:
Marvel is getting all of its properties back and rebooting them ("The Incredible Hulk" and the just-announced "Punisher", for example) so that they can exist the shared Avengers Movie Universe they are creating. Sony must have still have the rights for more Spider-man movies, so they're going to reboot it while they still can so that when the rights defer back to Marvel, Marvel will be less likely to reboot it for the third time in a decade and Sony can still bank on their existing movies instead of having them erased by what would amount to be the "Official Marvel" version.
This is only my theory, but I think it makes sense business-wise.
Almost. The story as I understand it is kind of the reverse - Sony's license stated that a Spidey film had to be in production by a certain date or the rights would revert back to Marvel, which is why they were trying to push Spidey 4 along. When Sam, Tobey and the rest walked because Sony was turning the film into another Spidey 3 by trying to cram a ton of stuff into it, Sony turned around and put the reboot concept into production.
This isn't about improving on what came before, nor is it related to Marvel's control over their other licenses. This is about Sony holding onto the Spidey license, plain and simple.