Proper marbling really comes from cows that are allowed to graze and roam. Think of it like this. Feedlot cows are slaughtered fairly young, fed huge amounts at a time and also bred for size. That means that large pockets of fat accumulate in short times and gets too thick which makes it tough. Not necessarily that it wont taste good, but flavour is in the fat and with cows that graze, the fat accumulates in smaller quantities over a much larger period of time which allows fat and muscle growth in its lifetime to be continuous until slaughter. Better flavour spread out. Also notice the colour difference below. I'd bet the steak above was slaughtered between 1.5-2 years where the one below was probably at 8 months. The meat colour is like halfway between beef and veal.
Amazing striploin
Feedlot striploin
If anyone says this is off topic, beef is Canadian politics so get with it.