Softbank-owned Sprint has reportedly changed its mind about buying T-Mobile US, according to the Wall Street Journal. It seems a bit surprising, given the amount of time and energy Softbank CEO Masayoshi Son has put into pursuing the fourth-largest wireless carrier in the US -- committing to buy the company for as much as $40 billion. That said, the alleged reasons behind the decision to withdraw a bid make sense: Sprint faces too much of a challenge getting the deal over the necessary regulatory hurdles. The company likely came to this conclusion after a long series of meetings with government officials, who reportedly expressed concern that such a deal would take a toll on competition in the US wireless industry. Sprint declined to comment, but the Journal reports that the carrier plans to make an announcement early tomorrow.