ThoughtsQuite a number of these writing systems - Vai, Bassa, Kpelle, Loma, and possibly Manenka - are in Liberia. Liberia is a very special case of colonization. In 1822 the United States of America set up Liberia as a homeland for freed slaves from the United States; it became an independent republic on July 26, 1847. So the natives of Liberia came into contact with English-speaking people who were black like themselves, although the freedmen were mostly not members of the local cultures. The freedmen knew about the white man's writing. Exactly how this affected the natives deserves further research.
Even more remarkable is the fact that the Bassa script is phonemic rather than syllabic. Outside the writing systems of the New Invention, all the world's phonemic systems, and many syllabic systems, ultimately developed from the Phoenician script - the only exception being the phonemic Korean script, which developed out of Chinese characters. A few of the resettled American slaves may have been literate in English. Did these few literates help develop the phonemic Bassa script? This question also deserves further research.
With all the different African scripts, one wonders why
there was only one Sequoyah in North America. That may be due
to the land areas involved in each case.