The reason is almost certainly related to the main plot of the new film and the new villains that we’ll be meeting played by Cate Blanchett and Jeff Goldblum.
Perhaps Loki only took advantage of an opportunity that presented itself when Odin vanished.
If we take Tom Hiddleston’s comments at face value it would seem that the idea here is that Doctor Strange will use the Time Stone to look back into the past and see exactly what transpired that led to Odin disappearing and Loki taking over.