Who decided Dick was short for Richard?
Folk in about the thirteenth century - there was a tradition of rhyming nicknames, especially of diminutives, hence Robert becomes Rob then Bob, William becomes Will and then Bill. Same with Rick. Back in the day it might just as easily be Hick. The need to do it was probably because different generations had the same forenames as their parents and had to distinguish between them. The unfortunate association (Twelve o'clock and still no sign of Dick) is much later, say late 19th century.
They Pelted Us With Rocks And Garbage
English rhyming slang At the time, people used fewer names, so there were more nicknames. So Richard, becomes Rich, Rick and Dick Edward becomes Ed, Ned, and Ted. Robert becomes Rob and Bob. Short, one syllable names.
Robert - Rob - Bob Margaret - Meg - Peggy Richard - Rick - Dick William - Will - Bill Elizabeth - Beth - Betty Nobody 'decided'; it's just the way names developed in older forms of English.
Same reason that John became Jack