Checkers is a simple game with a simple objective; eliminate the other player’s pieces. Even the process of doing so is simple, jump the other player’s pieces and start a collection. There is very little thought as to a defensive strategy and it is often more reactive than strategic.
Even though the objective of Chess is simply laid out — obtain the other player’s King — the process in which to obtain this goal is much more strategic. Chess requires a greater understanding of the ability of each of the 9 different pieces on the board and a risk assessment that takes longer than a simple scan of the board.