Efficiency through focus
Multitasking kills efficiency. The more work items in flight at any given time, the more context switching, which hinders their path to completion. That's why a key tenant of kanban is to limit the amount of work in progress (WIP). Work-in-progress limits highlight bottlenecks and backups in the team's process due to lack of focus, people, or skill sets.
For example, a typical software team might have four workflow states: to do, in progress, code review, and done. They could choose to set a WIP limit of 2 for the code review state. That might seem like a low limit, but there's good reason for it: code that hasn't been reviewed not only hasn't shipped yet, but may need significant re-work before it is ready to ship. So it's important to take action on code reviews right away, and setting a WIP limit helps the team hold themselves accountable to that. It forces the team to knock out those reviews before pulling in new work.