The North Atlantic Current is a warm current. It brings in warm and moist air masses along the west coast of Europe. It also provides relative stability in the temperatures without extremes in the summer and the winter.
The conditions that this ocean current provides contribute to a fertile soil, enough precipitation, and nice climate, so the farming has nice predispositions to develop in this areas.
The plants that are very needy of water, deep and fertile soil, and do not require a lot of Sun light thrive in this areas, so no wonder that this areas are heavily used for agriculture.