The ground in different bits of the country is made up of different types of soil and rock.
Some combinations are able to soak up more water than others.
If a place that cannot soak up much water is rained on heavily, there is nowhere for the water to go and flooding happens.
The ground can only soak up so much water, to a level called the water table. The water table is higher in certain areas of the country, making those locations much more at risk.