Germany practiced "Guerre de Course" (Commerce Warfare) because they were weak in battlefleet strength. They had a weak navy. Commerce Raiding (Guerre de Course) can be compared to the land war's version of "guerrilla warfare", in that a combatant which has no comparable army (a strong army) to fight it's opponents must often revert to guerrilla warfare.
America practiced both "Guerre de Course" & "Guerre de Escadre" (Fleet Warfare)." America (United States) was WWII's exception to all rules:
1. The US had unlimited manpower
2. The US had unlimited natural resources
3. The US had out-produced every nation in every product; both friend & foe alike
4. The US factories were not being bombed or attacked
5. WWII made America stronger/while it made all other nations weaker
Note: WWII also got America out of the great depression of the 1930s!