(Reuters) – Sprint Corp has dropped its bid to acquire No. 4 U.S. carrier T-Mobile U.S. Inc because the challenge of securing approval for the deal was too steep, the Wall Street Journal cited unidentified sources as saying. Sprint and its parent, SoftBank Corp, decided it would be too difficult to win approval from regulators, the Journal cited people familiar with the matter as saying. Sprint had been working on a deal with Deutsche Telekom AG, T-Mobiles majority owner.