From a recent WSJ article:
"As SpaceX deepened its ties with the national-security agencies in recent years, the company’s lawyers advised senior executives not to seek a higher security clearance for Musk that would give him access to details about sensitive programs SpaceX is involved in, according to people familiar with the matter.
The reason, these people said, was that Musk would have had to answer questions from the government about his contacts with foreign nationals and drug use previously reported by The Wall Street Journal. ...
The SpaceX lawyers began analyzing the risks of seeking a higher security clearance for Musk after the Journal reported in June of last year about Musk’s use of ketamine, according to one person familiar with the review. The lawyers and executives concluded that if SpaceX sought a higher security clearance, it would risk Musk being turned down, or worse, losing the top-secret clearance he already has."