I'm a bit confused too. There's a principled argument in Unicode for when a glyph gets its own codepoint vs when it's considered sufficient to use a combining form. I don't know Bengali at all so can't comment on this case, although given the character now is in Unicode I guess the argument changed over time. Somewhere buried in the Unicode Consortium notes is an explicit case for the inclusion / exclusion of this character, it'd be interesting to find it.