Because the copyright holders at the time were much smaller and less astute.
You try to get the rights for a TV show based on Frozen. Disney will probably not sell those rights to you at any price.
While I understand where you are coming from, it's obvious they would. They are a public company, responsible to the shareholders, yadda, yadda. It's another question whether it would make economical sense to pay such a price which would likely run into the tens of billions.
Even now, book adaptations into movies, series, etc happen all the time. Even Disney still lets various companies (including freaking Warner Bros) make video games of its various IPs.
The dynamic here isn't about being astute, it's about well-positioned for a given market. If Disney thought a Frozen show was a good idea, they'd produce it themselves (like they did for Tangled and various others). For markets where they don't have / don't want to have an internal talent pool (RIP LucasArts), they rent the license.
I think it is ridiculous to think it might happen. It is like asking Apple to sell you M3 chips. They will use it on their own devices in the same way that Disney could make that TV show end-to-end for years, if not a lifetime. Think in the Star Wars brand.
Simultaneously they partner with Lego and other companies for franchising.
You think Disney IP should be free for all to use? Just remember this would go both ways - Disney could take the work of any independent author or artist and use freely without crediting.
At least the older stuff should become public domain.
They're trying to have their cake and eat it.
It's not normal or sane or good for society that things created 70+ years ago are still locked up in a corporate vault.
Your conflict is self-created and can also be undone.
Disney bought the rights to Peter Pan from Paramount Pictures in 1938. At the time, Disney was a small fraction of the size of Paramount Pictures.
The original author of Bambi sold the motion picture rights to his film in 1936 for $1000 USD to a film producer...Disney was not the purchaser. Disney bought those rights from the producer for an undisclosed amount.
Disney bought the rights to Dumbo for $1000 plus royalties on the book sales. (At the time Dumbo was a very simple, very short story intended as promotional content for a new type of book called a "roll-a-book". It sold very few copies, and would have been a historical footnote if not for the unexpected success of the film.)
Disney bought the rights to Winnie the Pooh and paid royalties to the Milne family for decades, before shelling out $350 million to buy the entire rights in 2001. (Disney originally paid $1000 for the merchandising rights to the merchandising rights holder, it's not clear how much they paid for the film rights but given that the Pooh books were a huge commercial success, estimates are somewhere between $100k and $1million in 195x dollars.)
Using public domain stories enabled others to easily produce cheap knockoffs legally, and they did. So, for instance you you had direct-to-video, cheaply made versions of "Cinderella" which were an attempt to trick unsuspecting buyers who really wanted the Disney version.
When some work is still under copyright, that makes is a higher bar for knockoff producers than for Disney. Disney has more money, more and better lawyers, and is known for the quality of its work, which makes it in a better position to negotiate the rights.
Disney didn’t mind paying for the rights to adapt copyrighted material, like they did for Bambi, Dumbo, Peter Pan etc.