Why do some Sino-Tibetan languages have tones while others do not?

The presence or absence of tones in Sino-Tibetan languages is not the result of a single, uniform ancestral system, but rather a cyclical and independent development across different branches.123

Tonal systems often emerge as a compensatory mechanism for the loss of other phonetic features. Specifically, phonemic tones frequently evolve to replace the loss of voicing contrasts in syllable-initial consonants (such as the distinction between p- and b-) or the loss of syllable-final laryngeal consonants like -h or -ʔ.2

Because these changes occur at different times and places, tone systems do not show regular correspondences across the entire family. Instead, the development of tones is driven by universal phonetic tendencies in "tone-prone" monosyllabic languages. While some languages possess elaborate systems of six to eight tones, others remain nontonal or have only rudimentary systems. In some cases, such as in certain Tibetan dialects, tones are linked to stress or specific articulatory features at the end of a syllable. Ultimately, tones in this language family appear to "come and go" over time, making it unlikely that a stable tonal system existed at the Proto-Sino-Tibetan level.213