Depth of field is a function of the sensor size, the size of the aperture and focal length.
Film tends to have bigger image area, which means longer focal length, which gives shallower depth of field. (the bigger the film, the more resolution you have also)
This image sorta explains it: http://blog.berniesumption.com/wp-content/uploads/2010/10/do... and http://s.hswstatic.com/gif/illustration-how-aperture-works-2...
Because large CMOS/CCD sensors are expensive most digital sensors are smaller than film. most dSLRs are APS-C sized, which is notable smaller than 35mm or "full frame" which is again smaller than medium format or 120mm those digital cameras are still notably expensive ($45k+) and less sensitive compared to film. 35mm DSLRs destroy their film counterparts now. Especially the nikon d800/600 and the canon equivalents.