Until very recently, the technology that read/sensed these numbers was not commonly found at merchants. Banks had it, hastening the process of clearing checks once they were deposited.
More on ERMA.
http://ed-thelen.org/comp-hist/Dev-of-ERMA—LessonsFromHistory.pdf
In the early 1950s banking was faced with a paper-handing crisis. Banks were unable to keep on top of the rising number of checks and were unable to retain bookkeeping staff. Bank of America, then the largest bank in the world turned to Stanford Research Institute to develop an automated bookkeeping and proofing system. SRI and BofA worked together to create ERMA (electronic recording machine - accounting) and to develop the MICR (magnetic ink character recognition) check coding system.