There were a lot of Senators who said that Trump was guilty but voted not guilty because they thought the trial was unconstitutional.
Quoting from the Constitution:
“...no Person shall be convicted without the Concurrence of two-thirds of the Members present.”
The Senators who thought the trial was unconstitutional could have left and not been present.
If there were 15 Senators who thought this way and left, then Trump would have been convicted.