Why cable TV frequency range ends at 1 GHz?
One factor (but not the only one) is certainly "plant leakage".

Plant in this case means the "cable plant" which is an odd term, but like a factory is also known as a plant, the installed base of copper and cable is also an asset and is called plant.  Very strange.

Because there is so much cable plant in a system, they try to keep the costs down, all cables will leak RF and the higher the frequency the greater the chance.  They have to decide on a cut off point, and trade off against cost.  With more shielding and better connectors costing more.

Hopefully someone else will chime in with other reasons, perhaps historical, amplifier limitations perhaps.