Indianapolis has the nickname "Naptown" because throughout its history many people have viewed the city as a rather slow and not-so-exciting place. It also received that nickname in part because Indianapolis can be long and difficult to say with six syllables, while Nap Town is considerably more pithy.
It's believed that Indianapolis being called Nap Town was popularized by jazz musicians in the 1930s. In the 1960s local radio personalities were encouraged to use the nickname frequently in order to downplay it and its negative connotations. More recently, the growth of Indianapolis, as well as the fact that the city hosted the Super Bowl in 2012, have allowed the city to escape its reputation as a sleepy city.