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