Why is foot washing so uncommon nowadays In John Chapter 13, Jesus tells us to wash each other's feet, and that we will be blessed if we do so. Why do most churches do not practice this?
How did this relatively easy and straight-forward task not become common practice in Christendom?

This answer does point out there are references of early Christians practicing footwashing, and there are few denominations still practicing footwashings.

Why is this not as common as communion? (Note: I'm NOT looking for why it is not part of the communion.) 


  When he had finished washing their feet, he put on his clothes and
  returned to his place. “Do you understand what I have done for you?”
  he asked them. “You call me ‘Teacher’ and ‘Lord,’ and rightly so,
  for that is what I am. Now that I, your Lord and Teacher, have
  washed your feet, you also should wash one another’s feet. I have
  set you an example that you should do as I have done for you. Very
  truly I tell you, no servant is greater than his master, nor is a
  messenger greater than the one who sent him. Now that you know
  these things, you will be blessed if you do them.


Might it be a possibility that only John refers to footwashing and not the other Gospel writers? If we compare this to communion, communion is mentioned in all the Gospels except John.

EDIT: I do not accept "Our feet are more clean nowadays" as an answer. Even though this might be true, it is still needed to wash your feet once in a while.
It seems that the washing of feet was somewhat more popular in the Early Church and even extended into several centuries afterwards.

St benedict of Nursia (480-543) in his Rule for Monks which covers all aspects of monastic life includes a chapter on the reception of guests. 

Chapter 53 states that "the Abbot pour water on the hands of the guests; and both he and the whole community shall wash the feet of all the guests. After this washing let them say this verse: "We have received Thy mercy, O God, in the midst of Thy temple." Let great care and solicitude be shown particularly in the reception of the poor and of travelers, because it is in them that Christ is more especially received; for, as regards the rich, the very fear one has for them procures them honor." 

At what point this custom fell into disuse is not known, but Dom Paul Delatte OSB the third abbot of Solesmes (1848-1937) in his Commentary on the Rule has this to say about the washing of feet:


  The Abbot shall pour water on the hands of guests and wash their 
  feet. Because the Abbot holds the place of Christ in the monastery, 
  therefore is this function reserved to him, recalling the condescension 
  of Our Lord to His Apostles at the Last Supper and expressing Christian 
  humility and charity. In ancient times, to pour water on the hand of 
  those who were going to table was the act of a servant or disciple; 2 
  with St. Martin 3 it became the act of a monk wishing to honour his 
  guests ; and St. Benedict makes it a rule. This practice is still observed, 
  and takes place at the door of the refectory when the guest is first led in. 
  As to the washing of the feet, a regular element in the ritual of ancient 
  hospitality, it no longer agrees with our Western manners and has long 
  been suppressed; we must honour guests, not embarrass them.