I am not sure I understand your question correctly, but I try... :). If "mama" would have been in vocative (the form of address), you would need a comma before "mama" and a pause when you pronounce the sentence. I also think that if "mama" would have been in vocative, the place in sentence would regularly be at the beginning, i.e. the first word, but this is not mandatory.
If you remove "mama" in the vocative case, the sentence should still make sense. "Este probabil numai" has Romanian words, but they say nothing put together like this. :)
If your question was "why is the answer <It is probably only mother> not accepted?" (I don´t know it this is a correct sentence in English!): "only mother" would be "doar mamă” and not ”mama”.
I was not referring to ”mama” as a form of address (vocative case), but as a name for this person when talking about her, usually within the family, but sometimes among closer friends. I recently asked a Romanian friend ”Ce face mama?” and he knew that I meant his mother. In the example given, what I pictured was one of the children in the family saying to another that their mother was coming.