My parents were both teachers, though not in the US, and they point to:
+ A decline in respect for teachers and the institution of teaching
+ Lower qualifications required to become a teacher relative to the general population[0]
+ More classroom hours and larger class sizes
+ The previous factors plus lower pay have been driving a systemic brain drain for decades, which forms a positive feedback loop to drive respect down even further
+ Greater opportunities elsewhere, along with the removal of studentships and incentives, make teaching less useful as a socioeconomic stepping stone
+ Inability for students who don't want to be in school to leave and get a job
+ Parents who are uninvolved in discipline or who actively counter it
+ Inability to remove disruptive students from classrooms, which is getting worse over time, as alternative pathways to school are no longer viable and disabled schools are shutting down[1]
+ An increasingly hostile environment where clearly false allegations are "taken seriously" to the point of not supporting accused teachers even after they're exonerated[2]
+ Far fewer men in the classroom than women, which has (unfair) effects on the perception of teaching as a "caring profession"[3] and on collective bargaining, as well as practical matters like boys lacking role models, and a worsened ability to verbally (or even physically) stop really disruptive students[4]
[0] The actual requirements have gone up, but a bachelor's is relatively less prestigious than a diploma used to be. 50 years ago about 30% of people stayed in school, while it's up around 80% now. The US stats are very different, of course, and this might not apply there.
[1] Not that disabled students are all disruptive, but one historical way to remove or manage disruptive students was to slap an autism/ADHD diagnosis on them, qualifying them for a classroom helper or to go to a special school.
[2] This may just have been a single incident that happened to a family friend, but it was bad.
[3] E.g. an underpaid one.
[4] My parents talk more in terms of senior teachers helping with classroom control in a wing, but grudgingly admit that it occasionally helps if some of those senior teachers are large and look strong.