First of all - after the consecration, they are no longer “wafers and wine”, they have become the Body, Blood, Soul, and Divinity of Jesus Christ. IT IS HIM - you have been taught that, I hope.
One can receive only the Body of Christ, and because it is ENTIRELY and completely Him, there is no need to receive His Precious Blood as well. In many parishes, such as mine, the cup is not offered to the people, and that is perfectly fine.
As for why some people chose to skip receiving the Blood of Christ, it could be for a number of reasons. They feel receiving His Body is sufficient (which it is), they are in a hurry (sadly), they have a cold and do not want to share that with others, they could be ill and have a fear of catching someone else’s cold.
It is not required to receive Holy Communion under both species.
Please be sure to speak with your RCIA Director to ensure that you have a proper understanding of Holy Communion - they should be teaching you all these things, and be the first place you go right now, while you are in preparation to be received into the Church, for these types of questions.
~Liza