The reasons why certain planned movies didn't end up the way they were initially planned to go are myriad and as broad as directors and producers going to creative war, tax kick-backs disappearing and executives not believing that anyone would buy a female action figure. You know, like a doll. Those things that have sold countless millions across the world. Look, nobody's saying these decisions were ever right...
That last point about marketing is arguably the most important one at an executive level. While a film-maker is inherently concerned with delivering on their own vision, a studio has more than just an auteur on their mind. They're the faceless folk who wouldn't let Lord and Miller make their "wacky" Han Solo movie, who robbed us of Edgar Wright's Ant-Man, who killed the Jurassic Park sequel with the mutant human-dinosaur hybrid soldiers and who killed Zack Snyder's vision for the DCEU dead. They're not always wrong.
They're also the reason why Tim Burton was only a producer on Batman Forever and not the director, as he had initially been set to be. Famously, it was decided that Burton's vision for Batman was simply too dark and wasn't appealing enough to the key family demographic that would mean the difference between some hundreds of millions and LOTS of hundreds of millions.
Yes, Burton's Batman movies were brilliant, but they were a carnival of grotesques and monsters - including Batman himself - and part of the point of the lead was that he was anything but aspirational. This was not a hero you'd want to step into the shoes of, because the cape and cowl came with emotional wounds. That made him more interesting, but it didn't exactly make him a good toy marketing tool.
And these things matter.
3. The Batman Returns Toy Problem
The story has always been that Warner Bros bowed to toy partners because they argued that there was no way of selling toys based on Burton's monstrosities. To be fair, they absolutely couldn't have done so to kids (though the Burton tie-in toys are delightful and the crowning part of lots of more grown-up collectors' collections) and Warner Bros were in the business of making fast bucks in the 1990s. They still are, but their approach is different.
That era was a time of instant return without too much thought about legacy. Sure, sequels were a nice idea, but looking back at the biggest movies of the early to mid 90s, they were geared towards short-term gains and ticket sales. That's why huge movies like Twister are mostly just remembered vaguely.
The studio needed their lucrative marketing partners for the first two movies. It was very much a huge part of their strategy as materials released ahead of 1989's original Batman reveal extensively. As the LATimes reported in 1992, the key partners were McDonald's, Diet Coke and Choice Hotels and a massive 120 product tie-ins were released including "talking toothbrushes, roller skates, and, naturally, T-shirts... boxer shorts, sunglasses and throw pillows... beach towels, beanbag chairs, weightlifting gloves and, yes, mugs."