> I don't know the full answer. The original proposal for Ethiopic
> it as an abugida, so the answer isn't ignorance, but what the reason
> actually was, I don't know.

I believe this was the authors' idea of efficiency at the encoding
level. Not so friendly for rendering.

I've tried to follow the Abugida debates in recent years but find it
challenging. In large part because the "Abugida" definitions seems to
be in the eye of the beholder.

As to Ethiopic origins of the term "Abugida", it is a column wise
rotation of the Ge'ez (classic ethiopic) syllabary in the Hebrew order
(preserved in the first column):


The name comes from the sound values of the first 4 syllables of the
first column. The rotated syllabary is simply a learning aid for
students who learn the syllabary (in Halehame order) musically, the
rotation helps trip them up when they have to recite the letters
(think of how the "alphabet song" breaks down if you mix the letters
up). So in this sense the "Abugida" is a permuted syllabary table (or
if ethiopic is an "abugida" and not a syllabary then the Abugida is a
permuted "abugida" table...).

Cognitively (based on comments people make when they aren't thinking
about it), people seem to conceptualize ethiopic letter elements as
"syllables", the collection then a "syllabary". When discussing the
systematic nature of the syllable shapes, people may speak more in
"abugida" terms. Perhaps the best model depends on the intended
application. The alphabet is viewed as a subset of the syllabary.

Personally, I favor the description Kamal Mansour offered of Ethiopic
as as "open alpha-syllabary". The discussion of IM designed based on
syllabary vs abugida logic is interesting.