It is hard to avoid the conclusion that this essay has been very carefuly scripted and drafted precisely to delay the "unveiling" of this very immodest "modest proposal." It is important therefore to ask what Swift does in the lead up to the revelation of his plan. Swift is very careful to try and establish the voice and tone of the essay to present himself as a caring, concerned and earnest individual who sincerely wants to help alleviate the poverty and terrible famine in Ireland. The reference to numbers and statistics likewise presents his voice as credible. Note an example of how this works in practice:
I think it is agreed by all parties, that this prodigious number of children, in the arms, or on the backs, or at the heels of their mothers, and frequently of their fathers, is in the present deplorable state of the kingdom, a very great additional grievance; and therefore whoever could find out a fair, cheap, and easy method of making these children sound and useful members of the commonwealth would deserve so well of the public, as to have his statue set up for a preserver of the nation.
Note how Swift is trying to show his reasonable nature by appealing to "all parties" and identifying areas of commonality between them all. Having established himself as a reasonable and even caring speaker, who is concerned for the plight of the poor, the shock of reading his exact proposal is that much more exaggerated. Thus the proposal is delayed to lull us into a false sense of security before shocking us.