At the end of the last ice age, Britain formed the northwest corner of an icy continent. Warming climate exposed a vast continental shelf for humans to inhabit. Further warming and rising seas gradually flooded low-lying lands. Some 8,200 years ago, a catastrophic release of water from a North American glacial lake and a tsunami from a submarine landslide off Norway inundated whatever remained of Doggerland.

https://www.nationalgeographic.org/maps/doggerland/

Some guesses:
-A lack of shallow water
-Environmental impact concerns
-No longer economically viable to maintain
-Less demand for rural land
-Worries about sea level rise in the future