Certified GKH Gardening Expert
My guess is that they've found a better tasting feast elsewhere. Another possibility is that natural predators have come to visit, such as birds, ducks, etc or something that has eaten the eggs usually left by these slimy creatures. Have you perhaps used something scratchy in your soil this year?
Here's an interesting article from University of CA:
and one from GKH about what deters them: https://www.gardeningknowhow.com/plant-problems/pests/insects/slug-snail-eggs.htm