Because in the grim darkness of the far future, there can be only... Oh, wait. Wrong universe.Its a cliche of the cyberpunk genre. In the real world, the sun still rises, clouds dissipate, and people go on about their lives. Why should eternal darkness and rain be the setting for this game?
The reason people expect the lack of sun and perpetual rain to be the norm for CP is because - to them - it seems to be in tune with the bleak, dystopian future the setting is painting. If the future is crap, then the weather should be crap as well. It helps to build and reinforce a certain atmosphere. In the reverse the sun, or dawn, is often used as the metaphore for hope or something good in general.