One reason to focus on kihon in the early stages of kumite is the lower risk of injuries.
If you can lift you knee high enough to kick chudan, you'r less likely to hit someone in the privates.
Good kihon can also result in less fatigue, because strength isn't that necessairy to hit hard. This can also result in fewer injuries, because when fatigued things get sloppy.
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Ives