Why walking meditation is not mentioned in the ways of attending to mindfulness? Namo Buddhaya.

Buddha talks about walking meditation at several places in the suttas for example here. However he doesn't mention it in the Long discourse about the ways of attending to mindfulness. There are 4 ways of attending to mindfulness but none of them mention walking or walking meditation.

My questions are : 

Why doesn't Buddha mention about the walking or walking meditation in the long discourse about the ways of attending to mindfulness?

Is there any sutta where Buddha talks about the technique of walking meditation?
I think that walking is mentioned in DN 22 -- in the translation you referenced, it is this:


  Moreover, monks, a monk while going knows “I go”; or, standing he knows “I am standing”; or, sitting he knows “I am sitting”; or, while lying down he knows “I am lying down”; or, in whatever way his body is disposed, he knows it is (disposed) in that way.


Ven. Sujato's translation:


  Furthermore, when a mendicant is walking they know: ‘I am walking.’ When standing they know: ‘I am standing.’ When sitting they know: ‘I am sitting.’ And when lying down they know: ‘I am lying down.’
  Whatever posture their body is in, they know it.