Causes of low SNR
There are many things that can cause low SNR. If you put aside natural reasons such as attenuation caused by distance, most of the time problem lies in bad infrastructure. Bad cables (damaged shielding and exposed wires), ingress noise, crosstalk, impedance mismatches, bad connectors and micro-reflections, bad splitters and filters and stuff like that. It depends on many different things and it varies in real time.