Because, so long as the evidence against him was vague and intangible, it was very hard to disprove. 
Proving your guilt or innocence is very easy to do when someone makes a charge against you. 
mnli hypothesis: Proving your guilt or innocence is very easy to do when someone makes a charge against you.  premise: Because, so long as the evidence against him was vague and intangible, it was very hard to disprove. 