You misunderstand my question. Jesus instituted this sacrament with both bread and wine. Paul affirms this is to be the practice of the Church. Why is it no longer a common practice to receive both species? It’s not a question of receiving fullness of grace (though that seems a bit confusing as stated in the Catechism, and I am still curious how you can receive the “fullness of grace” by just receiving one species, but it’s “more complete” when you receive both. How can you be “more complete” if you’ve already received the “fullness of grace”?).
The reason cited for the single species versus both being the most common form is “for pastoral reasons.” This should not be too hard to imagine situations where requiring both could cause difficulty. If it’s one thing the Church is very consistent about, it is not making this sacrament into a sideshow. By that I mean, causing more or less attention focused on any one individual receiving it. So think about this… people take all kinds of prescription drugs for any number of reasons. Some of those medications lose their entire effective dose if taken with the slightest bit of alcohol. Some meds, like Antibuse, can cause one to become violently ill if alcohol is so much as sipped. Everyone under such treatment would be forced to forego the sacrament, or it could lead to focus on the individual as they publicly “deviate from the norm” in full view of a gathered congregation by “bypassing the cup” (if normally required of everyone else) as it were. That’s just one possibility; I’m sure there are many other good reasons for not turning sacramental celebration into circus.
The other part of your question, asking about how can something be “full” in one instance and “more complete” in another is explained very simply. You are making a presumption that both statements are referring to the exact same thing (grace), but they are not. The “full” refers to the Eucharistic grace, but the “more complete” refers to the “sign.” Remember, sacraments are outward, visible signs of the inner, invisible grace. So what the Church is saying is that while the grace in either case is identical, that outward sign is more complete in the case of both species.
The Catechism of the Catholic Church, 1390, spells this out: (Italics mine)
Since Christ is sacramentally present under each of the species, communion under the species of bread alone makes it possible to receive all the fruit of Eucharistic grace.
For pastoral reasons this manner of receiving communion has been legitimately established as the most common form in the Latin rite. But "the sign of communion is more complete when given under both kinds, since in that form the sign of the Eucharistic meal appears more clearly." This is the usual form of receiving communion in the Eastern rites.
If you compare this with some of the unusual circumstances where other sacraments are given, you will see there is a consistent emphasis on the inner grace as the most important part, while sometimes, out of necessity, letting the “usual” outward sign part of the sacrament be relaxed or even done away with.
Hope that helps you.