It’s lack of direct evidence. Trump operates like a mafia boss. No paper trail, always use euphemisms for direct action, use intermediaries for voice communication. That’s why everyone around him got caught and prosecuted, but he didn’t. The entire case would be hearsay interpretations. Federal guidelines wouldn’t let a prosecutor take that to trial. Usually in that case they go for intimidation to get a confession, which doesn’t work on a megalomaniac narcissist.