The economics for the company aren't great for people that make high frequency use of it. And I suspect that people that would pay for such a service nowadays would make good use of it.

Regarding the 5d guarantee -- I suspect that most disks would show up in 2-3 days, but if you're going to guarantee you'll need some buffer (as I think US Mail says first class is 1-5 days). And I think Netflix was just counting on it mostly being shorter (and may have even had distribution centers at some point in its history).