The Cubs set consists of ten cards. That is just half the number of Cubs from a year ago. No World Series or League Leader cards is the main reason.
Three of the ten that Topps included really have no business being shown as Cubs. Alex Avila, John Lackey, and Koji Uehara became free agents after the season and there wasn't much chance that any of the three would end up back in Chicago. If I ran Topps, I would decree that no free agents would be included in series one. I'd save them for series two or update, when they can be shown with their correct team.
The seven current Cubs that Topps included were good choices, all top line guys. The biggest surprise to me is that Kris Bryant will be in series two. He went from being card #1 in 2017 to at least #351 a year later.