It probably has to do with publishing rights. Authors may not want to allow digital publication without an actual print run, and once the initial print run ends

It probably has to do with publishing rights. Authors may not want to allow digital publication without an actual print run, and once the initial print run ends they lose the digital rights unless they do another print run. Or the digital publication rights may only be negotiated for a fixed period of time, or else require an ongoing fixed payment to retain, so it costs the publisher money to continue offering a digital version that they may not recoup without sufficient sales.