Because there is a sprint / T mobile merger that he wants badly and it would have tanked under the current regulatory system and Trump is promising to break down regulation so that his mega merger can go through.
http://www.cnbc.com/2016/12/06/sprin...ion-in-us.htmlIn August, Bloomberg reported that Son was still eyeing T-Mobile and would still like to merge it with Sprint. In 2014, the Japanese billionaire considered buying T-Mobile, but dropped the effort once U.S. officials signaled they would be against the potential merger.