Because ISPs are cheapskates and don't want to upgrade their networks and hardware. There are better exceptions, like Altice who are in the process of replacin

Because ISPs are cheapskates and don't want to upgrade their networks and hardware. There are better exceptions, like Altice who are in the process of replacing their coaxial network footprint with fiber optics. Interestingly, according to them it will actually cause them to save money in the long run due to lower maintenance costs for fiber optics. I guess other coaxial ISPs are too greedy for short term profits to look ahead.