The list is too long and the day is too short
A BDC rep has maybe two hundred dials in them on a good day. The list has two thousand names, unsorted, most of whom can't or won't buy right now. So the rep works top to bottom, leaves voicemails, gets ground down, and the good buyer at name 1,400 never gets a call.
Agent turnover
Turnover makes it worse. You hire and train, the reps burn out on the dialing, and they leave.
The knowledge walks out with them, and you start over.