Follow the money
"The waterfall model is much maligned, partly because it was badly implemented and explained, and for safety critical systems it is more appropriate".
It has been very well and clearly explained in many places, although it does tend to be more complicated than agile methods.
The waterfall model has been much maligned mainly because so much money and prestige was to be made out of inventing the "latest new new thing" - agile.
Just look at the number of semi-qualified hucksters trying to launch careers and make millions by talking about "agile", when they can barely spell it.
Ask people like Kent Beck, Ward Cunningham, Dave Thomas or Martin Fowler - who actually proposed and pioneered agile methods - and even they will agree that the movement has become greatly diluted by charlatans.