There is a period of time before LEOs make the information public, during which they try to determine if there is any evidence that the suspect (person-of-interest) has accomplices. They don't want to spook any possible associates by announcing that they know the suspect's name. They need to be able to put out BOLOs before tipping them off that heat is coming.
My guess is the cops (FBI et al) were fairly confident there was no evidence of anything but a single bad actor, so they released the I.D. to get the public's help in tracking him down (no associates to spook by doing it).
