I'm not entirely sure about this, but I think it is absent chiefly in herbivorous mammals. That would make sense if this were true -- and I remember it correctly -- since bile functions in fat absorption. In these animals, bile can still make its way to the duodenum; it's just not stored in the gall bladder.

Ditto on the "not sure." Plants contain fats, i.e. lipids, mainly in their seeds. Perhaps the absence of a bladder was constrained to those verts whose diet did not include, or could not digest, seeds. Or might the emergence of bile and its gall bladder store themselves be emerge from a shift in diet? Diagramming those verts with and without a gall bladder might yield insights.

Sm Banan:

There is no straightforward answer to your question but this link should provide you some basic insights:

https://medium.com/concepts-of-zoology-the-paradigm-shift-group/why-some-animals-dont-have-any-gallbladder-fe53cc44ee83

Best

Syed