Scrum has been very well marketed over the years, and many folks believe that Agile IS Scrum.
But, there is another reason:
When adopting Agile, most companies and the management tasked with the adoption, are both too inexperienced in the topic (those tasked typically come from inside the company and lack any actual Agile experience), and too afraid of failure.
The result is that they pick what has become the industry standard with the premise that if they do it by the book and it goes wrong then it is the fault of Agile being unsuitable/incompatible with the company rather than it being their fault.
My answer was always: “10,000 flies can’t be wrong!”
I have seen multiple billion dollar companies try this even when there was no way that pure Scrum could ever work in their corporate culture (highly regulated, too big to fail, regularly audited, all development outsourced and offshored to companies that claim to be both Six Sigma and fully Agile)...
I remember one company tried Agile in a major business unit and decided to abolish all IT and Dev management positions because “manager” was not a role defined in the Scrum guide...
You can imagine the reaction of teams tasked with the delivery of $100M Regulatoty Compliance systems to that one!
Needless to say, Agile adoption within the business unit was nonexistent, and the manager responsible still got a nice promotion!