Because Mr Oswald's claimed alibi had to be buried.
It wasn't a false statement.
That was not the reason it was written down. The reason it was written down was that Mr Oswald himself had said it.
So either Oswald is a complete looney tune and or an idiot, to make false statements that surely he knew would be proven false, or, he told the truth.
If Oswald had said nothing, and Hosty made up the statement, it could be only for one reason, to later paint Oswald as a liar. But if this were the motive, then surely Hosty would include the P.Parade in later typed draft. Therefore, Oswald made the statement and Hosty wrote it down initially, but then for some reason, Hosty decided not to record it later. Why?
If its a false statement made by a looney tune Oswald, that Hosty intially wrote down in handwritten note, but later omitted the statment, it can really be only for one reason: There was too much risk that someone of the many people NOT called to testify, might CONFIRM Oswalds statement.