And since the guy's name was PATrick not Padraig using D's instead of T's just reinforces the illiterate Irish stereotype.
The English language has a ton of stupid rules,(like how do you get Dick from Richard?) and this is just another on the long list.
Because it's not based on English "rrules" per se. it's a culturalistic thing. EVERY language has them.
When the Normans invaded England in 1066 they transformed not only politics, religion, society and language, but names as well. They brought their large stock of Germanic names, only Frenchified, reflecting their language.
The native Anglo-Saxons (now the lower classes) faced with an unfamiliar language and strange pronucnations. Often, they were uncomfortable with the Norman “R” found in names like Richard, Roger, and Robert, and Henry/Harry.
Another factor to consider is that most diminutives are coined when speaking to children, or when children are addressing one another. Sounds that many English-speaking children have trouble pronouncing the letter “R” as well as other sounds. The name Ouida, for example, came about a mispronunciation of the name Louisa. In names, the sounds of R is often switched for an L or a D. Often times it is dropped altogether.
Other times, a middle “r” would be swapped for an “l” or a “d”. R’s at the beginning of names fared little better, often being replaced with “Ds,” “Hs,” or even “Ns.” Robert gave up the nicknames Rob, but also Dob, Hob, Nob, and later, Bob. And Richard gave up the nicknames Rick, but also Dick and Hick, while Roger clocks in with Rodge, Dodge and Hodge.