In the following sentence, why 'は’, not 'を’、is used as 格助詞？？
The particle は can be combined with numerous other particles. For example, you may have already seen things like
However, in certain situations, the combination gets reduced as in
These are the only two cases I know of where the joined particle gets fully absorbed into は.
So the speaker wanted to say something like
But, in modern Japanese this isn't grammatical and the をは becomes just plain は.
In older documents (or documents using an older style of writing) をは isn't necessarily reduced in this manner but becomes をば instead. And, occasionally this pops up in some dialects.