press release says the new date " is driven by the belief of the creative team that this gives the show the appropriate time for delivery of the highest quality, premium edition of the first new “Star Trek” TV series in over a decade." Considering the special effects heavy show would need to have completed filming its first episode by at least the end of October to make a January premiere, the news comes as no surprise since casting has not even started. That estimate probably would cut things too close as whatever lead time would be eliminated by time include about three to four weeks of Holiday time off that most shows take. Net result there was no way they could have maintained a weekly release for 13 weeks if they had not even completed casting by this point as scheduling then becomes an issue due to lack of lead time.
While it will never be said, I suspect what really happened is Bryan Fuller's plate simply got too full. While prepping Discovery, he was also completing the first season of American Gods so the divided attention probably slowed down prep since his approval would be heavily needed for concept designs, set designs, casting choices, script rewrites, and a thousand other daily decisions that a new show requires. A new show is like a new born baby, every aspect of its care have to go through the parent (show runner), no matter how small. As the "baby" gets older, the parent doesn't necessarily have to be involved in every single little decision as others (in this case on set producers, writers, etc.) now have the guidelines and approval to make those decisions but getting that in place requires to building it and that is where they are at right now.
While fans may be disappointed, this may prove to benefit the show. The January to March schedule would have had the show competing against network and cable television as they not only started to premiere their second half of 2016-2017 shows but also be at the height of "sweeps" in February to goose ratings for the end of season in May. By waiting until May to premiere it on CBS, the network can more fully advertise the show and it will enjoy a mostly competition free summer to advertise and build an audience. It also means the first half season of cancellations will have concluded which would free up actors that might be perfect for the roles Fuller and company are creating. This does seem like a very old school way of thinking vs how streaming ratings work but keep in mind that CBS is holding the purse strings. The practically invented the old way of thinking so wrapping their heads around new approaches is going to take time and likely lessons learned the hard way. The hope is Discovery doesn't become an object lesson via an early cancellation due to their inexperience.