I put Santes but now I see in the Gweiadur that Sant is masculine and Santes feminine.
With many people of historical significance in Wales, the honorific usually comes after the name and is sometimes mutated. For example:
- Dewi Sant - ‘Dewi the Saint’ (but just plain ‘St David’ in English). Other saints are not accorded the same importance.
- Hywel Dda - Hywel the Good, an early king of nearly all of Wales and renowned for his efforts to try and create a unified Welsh legal code still known as Cyfraith Hywel.
- Rhodri Mawr - Rhodri the Great, a king of Gwynedd.
- Merfyn Frych - Merfyn the Freckled, Rhodri Mawr’s father.
- Barti Ddu - Black Bart, Bart the Dark, a 17/18th century pirate. The surname ‘Dee’ is perhaps an anglicised version of Ddu.
- Owain Lawgoch - Owain Red-hand, a 14th century claimant to be the Prince of Gwynedd and Prince of Wales.
Because Padrig has no special significance in Welsh history - he is just San Padrig (St Patrick), not in the same historical league as 'Dewi the Saint' in Wales.
Because of his status as the patron saint of Wales is the most likely explanation.
Also maybe because his saint's day, ie 'Saint David's Day' is known in Welsh as 'Dydd Gŵyl Dewi' (lit:- David's Festival Day) without the word for saint even in the phrase.