The second edition of the OED was printed in 1989. The third edition hasn't even completed the letter "R." Thus, by the time it reaches "Z," demand for the third edition in printed form may be so low that it will simply make no sense to publish it that way. In fact, according to the OUP, the digital version of the Oxford English Dictionary gets 2 million hits a month from subscribers, who pay $295 a year for the service in the U.S. Meanwhile, the current printed edition, all 20-volumes and 130 pounds of it, priced at $1,165, has sold only about 30,000 sets all told.
A statement from the OUP said, "At present we are experiencing increasing demand for the online product. However, a print version will certainly be considered if there is sufficient demand at the time of publication."
The OED was first published (in parts) beginning in 1884. The complete text was published in 1928.
Meanwhile, the online edition was launched in 2000. Obviously, it's far easier for OUP to update a digital edition than a printed one. While the OED hasn't been published in printed form since 1989, online editors put updates out every three months.
In December, the online version will be relaunched with its first major update since 2000. The new site will include a historical thesaurus to make cross-referencing easier.