The reason for the difference is that there is no efficient way to diagram the rule "W, X, and Y cannot all be selected together" using a Double-Not Arrow, although you could
diagram it using Double-Not Arrows. To do so, you would have to say:
That doesn't make a lot of sense to try to diagram when you can more easily use a Not Block to show that you can't have all three together. Any benefit of using the Double-Not Arrow is overcome by the relative of using the Not Block in this particular instance.
However, for Drill 5, in which the rule is simpler ("R and X cannot be selected together"), the benefits of using the Double-Not Arrow make that diagramming technique the better choice. Most people find that they have an easier time linking conditional rules and gaining inferences using the Double-Not Arrow than Not Blocks for rules such as this.
Please let me know if that helps.