Because cable TV was almost universally available and nearly as universally installed (at least in urbanized areas) before home internet access became common, cable is very common in US cities due to the infrastructure already existing.
And since available speeds far outpace DSL for fairly similar prices (or at least they did last time I looked at DSL), a lot of households subscribe to cable — especially where fiber isn’t yet available.
I think this is an important distinction that a lot of people don't realize.
When non-terrestial television became big, North America went largely with cable, while Europe went with satellites. Thus, the infrastructure was already in place for NorthAms to get high speed internet over their existing coax connections.
And since available speeds far outpace DSL for fairly similar prices (or at least they did last time I looked at DSL), a lot of households subscribe to cable — especially where fiber isn’t yet available.