In the book, “The Sherlock Files” by Guy Adams. I read that at first they were going to have the usual six hour-long episodes per season. However, BBC decided to commission the series for three 90-minute episodes instead. So they basically rewrote and remade the original pilot (which I haven’t seen) that had been used to sell the series to the network and titled it “A Study in Pink.” The rest as they say is history.
 
As frustrating as it is to have only three episodes every season, I do think that the hour and half long format makes the episodes more epic and gives them a greater sense of scope.