"The Office considered whether [the Trump Tower meeting] would establish a conspiracy to violate the foreign contributions ban... but the Office determined that the government would not be likely to obtain or sustain a conviction for two other reasons: first, the Office did not obtain admissible evidence likely to meet the government's burden to prove beyond a reasonable doubt that these individuals acted "willfully," i.e. with general knowledge of the illegality of their conduct; and second, the government would likely encounter difficulty in proving beyond a reasonable doubt that the value of the promised information exceeded the threshold for a criminal violation." (page 185-6)
OK, so that second part of the reason Don Jr. didn't get frog-marched was a technical thing. There was no way to assess the supposed value of whatever the Russians were supposedly offering, and hence no way to know if a violation would occur if the transaction occurred, but the hilarious part was the first reason there was no frog-march. Don Jr. and the rest of the gang were too fucking stupid to know it was illegal! That's not me saying it. That's Mueller!