I've been doing a little reading and it seems that before the English invasion only two native princes claimed the title of Prince of Wales (Llywelyn the Great and Dafydd ap Llywelyn) or controlled enough territory to make such a claim credible. What allowed these men to unite the people of Wales and why had no other ruler been able to do so previously?
I will probably not be able to do this in one post so I will break the issues at hand into the following rubrics.
The issues are:
Why was Wales rarely united under one ruler (inheritance and customs)?
What enabled a Welsh ruler to achieve success (success defined as personal hegemony over a large portion of Wales).
Why did only a couple of Welsh rulers claim the title 'Prince of Wales' (Welsh nomenclature)?
Now my expertise in Welsh history only permits me to discuss the period after c.1090 in any reasonable level of depth.
Operative Bibliography:
Patent Rolls of the Reign of Henry III: Preserved in the Public Record Office / Printed under the superintendence of the of the Deputy Keeper of Records, London, 1901-13.
(ed.) Dafydd Jenkins, The Laws of Hywel Dda, Law Texts from Medieval Wales, trans. Dafydd Jenkins, Dyfed, 1986.
(ed.) Thomas Jones, ‘‘Cronica de Wallia’ and other documents from Exeter Cathedral Library MS. 3514’ in Bulletin of the Board of Celtic Studies, v.12 (1940), 27-44. | Brut Y Tywysogyon: Peniarth MS. 20, Cardiff, 1941. | Brut Y Tywysogyon or The Chronicle of the Princes: Peniarth MS. 20, trans. T. Jones, Cardiff, 1952. | Brut Y Tywysogyon or The Chronicle of the Princes: Red Book of Hergest Version, trans. T. Jones, Cardiff, 1955. | Brenhinedd y Saesson or The Chronicle of the Saxons , trans. T. Jones, Cardiff, 1971.
(ed.) Huw Pryce, Acts of the Welsh Rulers, 1120-1283, with assistance from Charles Insley, Cardiff, 2005.
Ralph de Diceto, Opera Historica, ed. William Stubbs, 2 vol., London, 1876.
Roger of Howden, Gesta Regis Henrici Secundi Benedicti Abbatis, ed. W. Stubbs, 2 vol., London, 1867. | Chronica, ed. W. Stubbs, 4 vol., London, 1869.
J. Williams ab Ithel, Annales Cambriae, London, 1860.
Secondary:
David Carpenter, ‘Confederation not Domination: Welsh Political Culture in the Age of Gwynedd Imperialism’, in Wales and the Welsh in the Middle Ages, eds. R.A. Griffiths and P.R. Schofield, Cardiff, 2011, 20-28.
Rees Davies, Age of Conquest: Wales 1063-1415, Oxford, 2000.
Sir John Lloyd, A History of Wales, 3rd ed., 2 vol., London, 1939.
Max Lieberman, The Medieval March of Wales, Cambridge, 2011.
Susan Reynolds, ‘Secular Power and Authority in the Middle Ages’, in Power and Identity in the Middle Ages: Essays in Memory of Rees Davies, eds H. Pryce and J. Watts, Oxford, 2007, 12-22.
J. Beverley Smith, Llywelyn ap Gruffudd: Prince of Wales, Cardiff, 1998. | 'Dynastic Succession in medieval Wales', Bulletin of the Board of Celtic Studies, v.33 (1986), 199-232.
David Stephenson, 'The Supremacy in (Southern) Powys of Owain Fychan ap Madog: A Reconsideration', Cambrian Medieval Celtic Studies, no. 49 (Summer, 2005), 45-56.
W.L. Warren, Henry II, Berkley, 1977.