Microfilm is stable
Microfilm is an established technology. International technical standards for microfilm production are set and function well. Microfilm is a relatively "low-tech" product--all you really need to use it is a light source and a magnifying glass. Microfilm masters have an expected lifespan of 500 years. Microfilm is also relatively cheap to produce.
Digital media are changing fast
Digital preservation relies on very fluid technology, and significant innovations in digitizing occur on a frequent basis. The technology is currently changing too fast for standardization of format or equipment. A significant difference between digital formats and analog formats, such as microfilm, is that the output media for both is relatively stable, but the access mechanisms for digital media are in a state of rapid flux. You can read microfilm with a new machine or a twenty-year-old machine. But you cannot read a CD-ROM with a twenty-year-old machine unless the CD-ROM is the same age. Most personal computer systems are considered obsolete in two to five years. Computer vendors will make more money if than can sell people a new computer every few years, instead of having old computers remain viable for long periods of time. For example, the CD you bought today may stay in perfect shape for the next hundred years. But it in a hundred years, it is very unlikely that you will be using the same CD player you have today. Electronic equipment does not age gracefully. You may be fortunate enough to have your grandfather's gold watch; are your grandchildren going to be equally pleased to receive your first notebook computer?
In addition to the hardware, rapid changes in applications and operating systems make it difficult to digitize anything with confidence that the media, the hardware, and the software will all still be viable in the years to come. "Viable" does not just mean that it works, it is also essential that the product be supported. Some new technologies fail because they simply do not get a big enough share of the market, such as the Beta videotape format, and the Next computer. "Migrating" the data (making sure it stays compatible with new software) requires vigilance and a long-term commitment to preserving the data. There is also a significant financial commitment to maintain the hardware.
However, it is unquestionably true that digital technology offers much more flexibility in access. It is more productive to use one workstation to search on a topic, view the text, view the images, and follow all the links to related topics, than it is to look things up on a computer and then go to a microfilm reader to scroll through the film.
More information about the longevity of electronic media can be found at http://palimpsest.stanford.edu/bytopic/electronic-records/electronic-storage-media/. This reference is part of Conservation On-Line (CoOL), a web site maintained by Walter Henry at Stanford. Writings about specific formats are available at the cited page, as are responses to "Ensuring the Longevity of Digital Documents," an article by Jeff Rothenberg that appeared in the January 1995 issue of Scientific American.