Christ is present Body, Blood, Soul, and Divinity under each species. We are not required to receive under both species and do nothing wrong if we receive under only one species. We are merely afforded the *opportunity *to receive under both species with the current liturgical rubrics.
As to *why *someone might not communicate under both species-- illness goes to the top of the list. Some people have a phobia regarding a shared cup. Some people have allergies to wine. Some people are alcholics and should refrain from even a sip of wine. Some people just *prefer *receiving under one species as that is how they have always done it (only recently have both been available).
Really, it’s not our business.