The DTC stock borrow program was discontinued because it wasn't being used. Capitalism and enterprise made it obsolete. Private interests created their own stock borrow programs, which are faster and cheaper than DTC was providing. There are numerous other stock borrow programs still available.
"Usage of NSCC’s Stock Borrow Program has declined over the past few years. In 2007, NSCC borrowed a daily average of approximately $1.85 billion in market value at the close of each day from the approximately 21 Members that participated in the Stock Borrow Program. In October 2013, only three Members participated in the Stock Borrow Program and the average daily value borrowed at the close of day during that month was approximately $81 million. Usage of the program has continued to drop since the end of October 2013. Given the reduction in the use of the program, NSCC has determined that it is not economically efficient to maintain the service."
Members took their business to other programs, so DTC shut it down.