It's a long story that I'll try to simplify wherever I can. The blame can be put squarely onto the shoulders of a single movie.
A movie you never saw. Because they never actually made Spider-Man 4.
Marvel Studios has released a number of films based on their properties. Iron Man, The Incredible Hulk, Thor, and many more. But before Marvel had their own distribution company, the film rights to various characters would be sold to other companies.
|For better or worse.|
Spider-Man came out in 2002, Spider-Man 2 came out in 2004, and Spider-Man 3 came out in 2007.
Here's where the trouble begins. I'll go point-by-point for you. Because I care.
Point 1: Spider-Man 3 SUCKED.
Well, the film's degree of fail depends on who you ask. But the third Spider-Man film was disappointing for pretty much everyone involved. Fans, Raimi, Sony; they were all disappointed by the resulting film in some way. As such, Sony were clamoring for a fourth film ASAP (to squeeze every dollar they could out of an apparently-failing brand) while Sam Raimi wanted to make sure he had a good script before production began.
Sony: "We need a film now!"
Raimi: "How about we make a film that doesn't suck?"
Sony: "How could it possibly suck? We're already lining up John-freakin'-Malkovich as the Vulture."
Raimi: "Hmmm... How do you guys feel about Anne Hathaway as Black Cat?"
|Seriously, look it up. Hindsight, huh?|
After some time went by without any progress made on Sony's next Spider-Man film, Marvel had opened up their own distribution company and made 2008's Iron Man a surprisingly big hit. Sony only had the rights for film adaptations of Spider-Man as long as they were making movies. So things started getting rushed. They started considering filming two sequels at once, LOTR-style, the script was rewritten in 2008 and 2009, and the release date of May 6, 2011 was getting closer and closer. With basically nothing being done, time was quickly running out for Sony.
|"Join us, spider-Man."|
Long story short (too late), this brought up issues with their deal with Marvel. In order to rake in that sweet, sweet Spidey cash, Sony would have to fork something over to Marvel....
Point 3: The Sacrificial Spider-Man.
Sony also had the rights to make an animated TV adaptation of Spider-Man. It was called The Spectacular Spider-Man. Long story short, Sony traded over the rights to Spectacular Spider-Man to Marvel in order to ensure that The Amazing Spider-Man could begin production with more freedom and less Marvel breathing down their neck.
So that means that Marvel could keep making Spectacular Spider-Man but chose not to because they wanted to make Ultimate Spider-Man, right?
Point 4: Legal Crap.
Greg Weisman (the dude responsible for Gargoyles, Young Justice, and Spectacular Spider-Man) put it best when he responded in a forum post.
But to summarize his comprehensive answer, while Marvel had the sole rights to create or distribute a Spider-Man cartoon, all the character designs, storyline, music, etc. to Spectacular Spider-Man specifically was still owned by Sony.
In order to continue Spectacular Spider-Man, Disney would have to keep paying Sony money. So they just decided to start from scratch.
So basically, for all you tldr's out there, Disney didn't cancel SSM, they just didn't want to pay Sony the necessary money to continue it themselves. And they took this opportunity to create not just a new Spider-Man cartoon, but the first step to the increasingly-muddled timeline of the Marvel Animated Universe.
So that's all the legal crap. In Part 2, we'll take a look at the non-legal crap stuff.