Kaikaku is revolutionary change. Where kaizen is generally evolutionary in nature, kaikaku requires radical shifts in thinking.
Revolutionary changes tend to be far more challenging in nature and much less common than incremental improvement. Because of the broad, sweeping changes that kaikaku brings, it is generally driven by higher level leaders, and requires the commitment of greater continuous improvement resources than everyday improvements. It can also be hard for frontline employees to embrace the major changes that kaikaku brings.
Kaikaku also has the characteristic of being novel. Most kaizen improvements are variations of something that has been done before. While they entail creativity, there is no breakthrough thinking in moving machines closer to each other or reducing setup time. But with kaikaku, there is often no blueprint to follow. While that makes it harder to do, it also has the advantage that competitors are unlikely to be doing the same thing.