Modern Sudoku was only invented around 1979, and not widespread for another 10+ years.
Source: Sudoku on Wikipedia
Had the algorithms been developed immediately after Sudoku became widespread, it would have only been about 25 years before the dates you mention. Hardly time for them to be well-known for decades
When designed for speed rather than adherence to human solving process, and with the notable exception of those based on exact cover, most of the Sudoku solving algorithms I've seen are functionally equivalent to DPLL from 1962.
Usually the correspondence is not explicit and the representation is not CNF. Instead the representation and the algorithm are ...
Well, I have two points to make.
First, Crook's algorithm has more to it than the part you've described here. Specifically, if the part that you've described fails to solve the puzzle, then the next step of Crook's algorithm is to select an arbitrary unsolved cell, pick an arbitrary possible value for it, and attempt to solve the puzzle under the assumption ...