There were so many well-directed films in 2018. That the Academy only picks five requires the snubbing of plenty of quality films. While the Best Director category logically follows from the Best Picture race, the expansion of Best Picture to a possible 10 films makes it harder to narrow down the Best Director race.
Any director whose film nabs a Best Picture nomination is in the hunt for a Best Director nod. Beyond those nominees, there is not much else in contention. And the Directors Guild Award nominations will clear up any confusion over the frontrunners. Aside from the weird year of 2012, the 2010s DGA and Academy Award nominations were almost the same. Each year, just one slot was different.