Heyooo. Great question, albeit basic. It’s due to the temperature over time.

As I’m sure you know, winter is winter and colder, because due to the earths orbit and tilt, we are further away from the sun, and we get less sunlight in a rotation than we would get in summer.

Clouds are water vapor and in summer, a warm ground will cause water to evaporate and show up as nice puffy clouds in the sky and then perhaps dissipate all together. In periods with colder temperatures, the reverse actually happens. The ground itself is cold and wet, and the air is more stable and the ironically warmer air higher up will be stable to where clouds form that are huge and flat, squeezed between these layers. They also last a lot longer since no sharp heat from the sun hits it.

In fact, since they become big and flat, even less sunlight gets in and so they make this layer that keeps all that “dew point” water vapor to stay locked in. Result, we get more rain. If it was colder it would just freeze and come down as snow, but since this has been a mild winter this year, all we’re getting is rain since it doesn’t freeze properly.

Does that make sense? Hope I helped out and didn’t make it sound even more confusing.