The satellite radio operator's shares have tanked almost 70% in the past five years.Sirius XM (SIRI +3.01%) is the only satellite radio operator in the U.S. While this might seem like a favorable competitive position to be in, it hasn't helped the shares in the slightest. They have tanked 67% in the past five years (as of Nov. 19). This certainly isn't an encouraging trend.There might be one critical factor hindering this business. Investors shouldn't think about buying this mid-cap stock until this big thi ...