How are phono cables different (related to hum issue)?

Wrestling with frustrating humming for weeks on a new TT. Finally inserted the cheap stock phono cable, and the hum is nearly gone. Was using spendy phono cables bought separately prior. So what's the sitch here? Shielding? Cheaper = carrying less signal? I was using the ground wire with both (and tried without, twas worse). Glad the hum is gone, the sound is duller but removing the hum makes it almost worth it. Of course now I need the color back, but not the hum.