Nexus kinda ran out of numbers, since they already used 7 for the tablet.
Google experimented for a while with weird model numbers like 5X and 6P, but seems to have decided against continuing it. It's the version wars all over again. Who wants to be stuck at 5 and 6 when Apple is at 7, Samsung will soon return with an 8, and Microsoft already played the "we'll skip ahead to 10" card?