Why is 'attention' ('manasikara') included within 'nama-rupa'? In respect to Dependent Origination, the Pali suttas state: 


  Vedanā, saññā, cetanā, phasso, manasikāro - idaṃ vuccatāvuso nāmaṃ. Cattāri ca mahābhūtāni catunnañca mahābhūtānaṃ upādāya rūpaṃ. Idaṃ
  vuccatāvuso rūpaṃ. Iti idañca nāmaṃ idañca rūpaṃ - idaṃ vuccatāvuso
  nāmarūpaṃ.
  
  Feeling, perception, volition, contact and attention — these are called mentality. The four great elements and the material form
  derived from the four great elements — these are called materiality.


Alternate translation:


  Feeling, perception, intention, contact &amp; attention: This is called name. The four great elements and the form dependent on the four great
  elements: This is called form. This name &amp; this form are called
  name-&amp;-form.


Why do we think 'attention' ('manasikara') is included within 'nama-rupa'? 



Note: I personally do not have a definite answer to this question, apart from personal speculation therefore I have asked the question for people to think about &amp; answer.
There is this hierarchy in the teaching. One such is in the Nama…..  of the  rupa (material phenomena) &amp; nama (mental phenomena).  Nama is broken down into Vedana (feeling) , Sañña (perception),  cetanā (mental activities),  Passa (contact) , and manasikāra (mental advertence). Buddha placed Passa &amp; Manasikaara to the end for a reason. For the three things: feeling, perception and volition (vedana, sañña, and cetana) to be born contact must be there. Dependent on contact only manasikāra (mental advertence) happens. 

Vedanā, saññā, cetanā multiple series of mental events are rapidly arising and passing away from moment to moment. Only if contact happens -  Passa paccaya vedanā, saññā, cetanā….  with vedanā as condition, Tanhā arises.  Passa (Pāli) is a Buddhist term meaning "contact" or "touching”.  Dependent on contact only manasikāra (mental advertence) happens. 

In the case of an arahant the manasikāra is in accordance with the Dhamma. They live in the timeless present. For an Arahant in reference to the seen, there will be only the seen. In reference to the heard, only the heard. In reference to the sensed, only the sensed. In reference to the cognized, only the cognized. For him it is only “Passa”, whereas for you and me, it is ‘sampassa’ (san+passa). 

“San’ is basically the term for “good and bad things we acquire”. When we are thinking about an experience in the past or worrying about a future event, it all happens in the present moment. They are all activity of nāma-rūpa happening only in the present. From this ‘sampassa’ (san+passa) comes Ayonisomanasikāra.

Thus Manasikāra simply means ‘attention’. This in essence is the ability to be aware of the activity of the senses and the origin of mental phenomena, as it happens. The activity of the six senses. (1) Eye and forms (objects), (2) ear and sounds, (3) nose and odours, (4) tongue and tastes, (5) body and tactile objects, (6) mind and mental phenomena (mind objects). This happens both in you and me, as well as in Arahants.

In Arahants there is cetanā. In you and me it is san+ cetanā.  There is a distinction between sankhara and abhisankhara. Sankhara involves everything that we do to live in “this world”; these include breathing, walking, eating, etc. Even an Arahant has to be engaged in sankhara until Parinibbana or death. The prefix “abhi” means “stronger” or “coarse”. Sankhara become abhisankhara by engaging in the “wheeling process”. The sansaric process or the rebirth process is fueled by abhisankhara.