Why do Hadza and Sandawe differ from Southern African Khoisan languages in word structure?

Hadza and Sandawe differ from Southern African Khoisan languages in word structure because they do not adhere to the strict phonological rules that govern the Southern African group.1

In Southern African Khoisan languages, there are rigid constraints on the placement of sounds within a word:
* Initial position: All clicks and most nonclick consonants must appear at the beginning of a word, followed by a vowel.1
* Medial position: Only a limited set of consonants (such as b, m, n, l, and r) are permitted between vowels.1
* Final position: If a word ends in a consonant, it is restricted to m or n (with rare exceptions for specific grammatical suffixes in Khoe languages).1

Hadza and Sandawe deviate completely from these restrictions. Furthermore, while Southern African Khoisan languages show a strong bias toward clicks—with click words often outnumbering nonclick words by a ratio of 7:1 or 8:1—Hadza is characterized by a different history in which nonclick words actually outnumber click words by 7:1. These structural differences reinforce the conclusion that Hadza and Sandawe have distinct historical developments compared to the Southern African groups.1