Now that we have all thrown our bricks at each other, lets take a look at the physical process of Communion in terms of movement.
JLW, if I recall, you are form Gresham. If you go to Mass at Holy Rosary, by the Coliseum, you will see the following: people start kneeling at one end of the altar rail, with the next and the next filling in positions to the left of the first communicant. The priest starts distributing Communion, and as he has moved down the row three to four persons, the first communicant is getting up to return to their seat. We now have two groups of peole in motion behind anyone receiving; those going to kneel to the left, and those from the right moving towards their seats. It makes for constant movement, with people dodging one another as they move either to or from the Communion rail.
One of the first things that standing to receive does is reduce the amount of movement. Constant movements by a group of people does not add reverence to the process, but instead adds distraction.
The second thing it does is lengthen the time it takes to distribute Communion to the second (and third, etc) person receiving, as each has to move up to the priest or EMHC. A priest distributing Communion to those kneeling can distibute it faster than people can get to a spot and kneel. In fact, with everyone kneeling already before he starts down the rail, he can distribute a Host about every second or second and a half.
So the second thing that receiving standing does is slow down the individual distribution. Lengthening the time it takes to distribute Communion to each individual adds to the reverence of the process.
Standing also makes it physically much easier to distribute Communion under both species. It is physically more difficult to take the Chalice and receive if the altar rail is right at your elbow height or higher. When standing, nothing is going to block the movement of your forearm or elbow, causing a spill.
While kneeling is a sign of adoration, standing is a sign of respect. The bow before reception is also a sign of adoration. Further, no one is denied the opportunity to kneel in adoration when they have returned to their seat, and I find that most people do kneel after reception. For those who insist that they want to kneel in adoration, that is not being denied; it is simply being moved to a time about thirty to 45 seconds later, if that much.
It also makes for poor liturgical process to be moving from one posture to another, then to another; in other words, bouncing around up and down might be appropriate for dance, but I’m not particularly big on liturgical dance. Prior to the change in the GIRM, we were kneeling during the Eucharistic prayer, then standing, then kneeling for the “Lord I am not worthy”, then standing to go to Communion, then kneeling individually (with the altar rail resembling the “wave” at a ballgame; one end getting up while the other was in the process of kneeling down), getting up, going back to our seat, then kneeling down again.
There is still a good bit of movement from the entrance procession to the beginning of the Eucharistic prayer, but it generally occurs over a longer period of time than from the end of the Eucharistic prayer to Communion.
So, receiving Communion standing results in a reduction of distracting motion, is a sign of respect, with a bow being a sign of adoration, makes it easier to receive under both species, lenghtens the time of distribution, and becasue of the lengthend time of distribution, acts to increase reverence in general.
Have I seen individuals receive standing who seem not to show much reverence? Yes, and I have also seen individuals not show much reverence when kneeling. The reverence an individual shows is much more related to the catechesis (or lack) which they have received than whether they kneel or receive standing.
I would not be the least surprised if there are other reasons we receive standing, but I suspect that the above probably have more to do with why the norm is standing than how many people have a problem kneeling.