First, because they didn't have the numbers in Congress to stop LBJ. Run a search on the partisan makeup on Congress in that era. Only by teaming with southern Democrats could the Republicans have an impact.
Second, Goldwater's defeat was the repudiation of everything he stood for. Only when the country hit rock bottom, crashed through that bottom, and found a whole new bottom in 1980 did the political calculus change. Johnson's victory gave him the opportunity to turn the country left. His goal was to be remembered as the second FDR. The country, thanks to the Mainstream Media, bought everything he was pushing.
But I'm just a nut fallen off the tree (wnk).