Ultimate Marvel only happened because, due to Bob Harras and Mark Power's increasing power tripping and general micromanaging (along with hiring Claremont back and basically allowing him to engage in some of his worst excesses), they managed to derail the X-Men franchise line to the point that they saw zero sales bump from the first X-Men film. And with the Spider-Man movie coming up and sales of the Spider-Man line being similarly in the toilet, Harras was told to pitch a line of new reader friendly books for fans of the movies.
However, Joe Quesada was already angling for a coup and managed an angle to get control over the Ultimate line AND provide a final straw for getting rid of Harras. While Harras wanted to treat the books as throwaways and give them to his cronies (Terry Kavanaugh for Ultimate X-Men, Howard Mackie for Ultimate Spider-Man); Quesada pitched hiring Brian Bendis and Mark Millar (huge names at the time) for the books instead and for them being given a huge push, with TPB/HC releases and heavy promotional push in all of the fan magazines and online.
Also, the Ultimate line had huge problems: Spider-Man was super-decompressed (to the point that you had issues where NOTHING happened) and basically became Bendis's persona fiefdom, propped up solely by the fact that Bendis was allowed to use Mary Jane as Peter's girlfriend (and dumping her the moment the embargo on her was lifted in the main book, replacing her with his preferred love interest, Kitty Pryde), editorial interference (Green Goblin being a Hulk, Venom appearing) and the fact that it quickly became locked out to casual readers due to the serialized format.
Ultimate X-Men was tainted by Millar's super edginess and his treatment of Wolverine, making him a wholesale villain who due to editorial mandate was "good". Also, Ult X-Men varied from decompressed as hell and super rushed, and once Millar left, you had a cavalcade of writers each rushing to introduce their pet character and stories and no real stability. Not to mention the edginess (Nightcrawler holding Dazzler prisoner to make her love him, Longshot as a murderer who butchered a man for sleeping with his wife, Deadpool as a mutant hating bigot who's sole character trait was how much he loved slaughtering mutants). And Ultimates was handcuffed by Bryan Hitch being a lazy fuck with massive gaps between issues.
Within a couple of years, the Ultimate line was reduced to being Millar and Bendis's personal sandbox that they blocked all other writers from playing in, then leveraged their success to highjack the main books of the Marvel line. Millar eventually left the line, but Bendis then seized complete control: resulting in garbage like Peter Parker being murdered (because that was the best he could hope for, given the arc Bendis gave him where Fury found out his secret identity and gleefully proclaimed whenever they met, that once Peter turned 18, he would lose all rights and liberty and become the property of the US government via illegal enslavement with Fury sending him to kill and worse, for him) and replaced him with Miles.
Marvel DID realize that they fucked up and created the Marvel Adventure line, with Jeff Parker (a no-name at the time) as it's main writer. But no one cared about it save for giantess fetishists on 4chan, as Marvel refused to kill the Ultimate line to replace it outright nor did they give the Marvel Adventure line a proper promotional push (though they did acknowledge the fatal writing flaws of the Ultimate line, as the Marvel Adventure line was strictly one off stories and Parker explicitly instructed not to be edgy or worry about serialization in his work). But the Marvel Adventure line eventually petered out.
Also, doing explicit tie-ins for the movie is a no-go because Bendis poisoned relations between the movie and comic sides and because the movie people have made it clear they don't want the comic folks meddling inserting their own lore into the franchise. That is why the tie-ins are treated as dispossible fluff and only get released, once the finished film is completed and they know what they can and can't do. And even if they did, you don't know what things will look like between movies and the overall direction of the franchises (Transformers had a huge problem with this for the Michael Bay film tie-in comics; you had writers writing stuff, then having to pretty much retcon EVERYTHING they wrote once they found out the plot of the next film and how it invalidated everything they wrote beforehand).
It was actually around 32-33 issues.
And there are a huge number of asshats in the comic industry who believe, in spite of what fans feel, that moving Spider-Man out of high school was a huge mistake and that Stan Lee was mentally exceptional for daring to do something like that to the character (IE aging Peter in real time).