That study has many holes and misnomers. Data in Europe may be, on average, less expensive on the surface. But US carriers essentially allow free roaming across the US whereas in Europe they catch mobile consumers with higher prices by charging roaming fees when traveling across Europe with mobile service from a national carrier.
The article also states that "In 2000, the two largest mobile phone carriers in the U.S. controlled about 35 percent of the market, while in early 2015, they controlled more than 65 percent." This is a result of the buyouts of the regional carriers, who weren't competitive with the national carriers. Thus the increased control by the national carriers, in this instance, isn't a result of less competition as the study implies.