I disagree with Simplesense’s theory that it is all political – though if we were just comparing the siyum alone, it might be a factor, The Dirshu siyum speeches were almost entirely in Yiddish, which possibly precludes a lot of non-Chassidim.
If you read back to what I said earlier, it is not just this siyum. As APY guessed, the participants in the Dirshu Kinyan Torah program in NY City are mostly Chassidim. I don’t know exact figures, but I say this from observations I personally noted at testing sites and and past Dirshu Shabbatons – where there was no Agudah competition.