What primary evidence led older studies to suggest the kiwi was a close cousin of the extinct Moa?

Geographical proximity and the shared trait of flightlessness

Prior to deep genetic sequencing becoming widely available and reliable, scientific understanding of evolutionary relationships often relied heavily on observable physical traits (morphology) and geographic location. Because both the kiwi and the extinct Moa were large, flightless birds endemic to New Zealand, and both islands were remnants of the supercontinent Gondwana, the intuitive scientific assumption was that they shared a relatively recent common ancestor. This model suggested a close evolutionary affinity based on shared island isolation and the simultaneous loss of flight in an environment lacking ground predators, a pattern that modern molecular biology has since shown to be misleading regarding their deep ancestry.