Why do some critics believe that Sylvia Plath's poem "Daddy" is not about her father, but about her experience with depression instead?

After Wimsatt and Beardsley's views on intentional fallacy, and its culmination into "death of the author" it is but natural that critics  have turned their attention away from biographical criticism. Any creative writing is disengaged from writer's background, especially, the biographical material. So, 'Daddy' also has to be looked through that lens.      

 She is certain thather flesh will soon be restored to her face after having been ... She does notunderestimate his concern, but is bothered by how he picks through her ashes. ... and it has drawn attention for its use of Holocaust imagery, similar to "Daddy.

Because she ended up killing herself and had had problems with her unfaithful and cruel husband

I have only recently read this poem, but I can totally see why it might be seen to be more about depression than about her father. However, I would argue that it does not necessarily have to be either this or that. You see, there could very well be numerous factors, feelings, emotions or motives that all together contribute to the creation of a certain work of art or literature. As such, to decide that a certain poem is more about this idea, so to say, than that idea can be rather misleading as well as reductionist.

One more perspective that I personally always like to bring up is that generally speaking, the interpretation of a certain work of art or literature -- as with almost any other text or narrative -- need not be strictly tied to the original intention of its author. This idea is most exquisitely explained in Roland Barthes' classic essay "La Mort de l'Auteur" (The Death of The Author).

well the poem is full of ideas and thoughts and belongs to many critical schools so if we want to talk about the poem from a feminist point of view its unadulterated anger and revolution towards  men in general the poem full of contradiction and she put this contradictions in purpose it was not spontaneous at all i mean after all men are full of contradictions and she reflects there nature in the poem