Waterfall was literally presented by its "creator" (Winston W. Royce) back in 1970 as an example of how not to manage a software project due to the lack of testing and validation until all implementation is complete (meaning, no integration or regression testing is performed as features are added during initial construction). This is also kind of the source of the axiom that, "The first 80% of the project takes 80% of the time, and the last 20% of the project also takes 80% of the time," in reference to the prevalence of budget overruns and missed deadlines/estimates once integration testing actually starts.

It's fine for trivial projects, and iterative methodologies (e.g., Agile/Agile variants, XP, etc.) use sort of a mini-waterfall phase on a per-feature basis. You're still performing the same activities, and often in the same sequence; you just toss out the rigidity of only performing each of those activities once for the entire project and thus introducing a fuckton of risk. Unfortunately, Agile became a weird cult religion at some point and a lot of managers are more interested in holding constant meetings than letting developers build software. Honestly, it has been hilarious watching my own IT org try to adopt some semblance of Agile principles while absolutely not changing their mentality or approach to anything; it's like watching a monkey sodomizing a football, but like, with my paycheck. I hate it here. Send help. Or nukes.