Because when we think of an Eastern patrimony the first things that come to mind are the externals- the particular form of the Divine Liturgy and customs that obviously distinguish it. Only after we have confronted that aspect of Eastern Christianity do we normally bother to investigate the spiritual methods and attitudes that lie behind the rite- the particular forms of prayer, the writings of the spiritual giants. This is part of the problem at the moment. I suspect that when some commentators are thinking of Anglican Patrimony they are not going beyond the externals of psalm chants, surplices and Tudor English all of which, by the way, have disappeared in the bigger world of Anglicanism. But what of the spiritual patrimony that might lie behind these externals?
If there is an Anglican Patrimony, and I think there is, it has to be understood as part of a part of the Western Spiritual Patrimony. It's externals are essentially Western, the spiritual practices, at least in their classical form and stripped of counter Reformation developments, contain a fading memory of what spiritual life in England was like before the Protestant Revolution. At the heart of this is a three pronged spirituality balancing personal prayer, liturgical prayer, and sacramental life. Personal prayer in the spiritual writings at there zenith in the 14th century, liturgical prayer in the maintenance of parts of the Divine Office for all the faithful, and sacramental life in the outward forms that survived. For a more detailed description of this Martin Thornton's English Spirituality is well worth a read.