[gioele]

Unicode is not about what you would like to use, but about what has been used in the past. New characters are avoid as much as possible and old characters are allocated a codepoint only if you can show that they have been extensively used and that they are really different from other similarly looking glyphs that already have a codepoint.

Probably nobody has found enough small alphas used in subscripts and there were no electronic encodings that included it so it has never been considered for inclusion.