I say it's confidence. The thing about being Modern Orthodox is that you know where you stand. At the end of the day, the Torah is the arbiter of your interactions and you can sin, you can watch television and you can go to the movies and you know that you are shomer shabbos, shomer mitzvot, all those good things. But with Hasidic Jews and Haredi Jews (except for Chabad which is its own meshuggas) there is a real fear at work. If they let their young people go out into the world, their young people might start judging the community as a racist elitist little enclave. So they continually inveigh against the evil social order that is best represented by ahem "modern" Orthodox Jews (their way of saying it) because they know that most of the people in the community are "failing to live up to their high moral standards", ie. going to strip clubs. What happens is that Chasidic Jews absorb the elitism, the racism, the ingrained misogyny and the superiority/inferiority complex and they spend years being creepy and if they leave it (go off the derech if you will) they aren't going to become Modern Orthodox because they have been trained to believe that regular Orthodox Jews are on the precipice and liable to fall into the haskalah with just a little push (after all, there are some shuls where women are "allowed" to make announcements and women are on a higher spiritual plane so why let them worry their empty little heads with a chilul hashem that could possibly drive the men who aren't drunk crazy with lust?)
So naturally, the Hasidic and the Haredi Jews are against Modern Orthodox because it provides a sane alternative to their messhugas. And naturally MO Jews invite them to speak because maybe they might have something insightful to say about the Torah since several are studying 8 hours a day (or just singing the words loudly without thinking about them) and regardless, they are a curiosity - like baboons showing off their big red asses to the world.
Of course, the baboons probably have better insights into Torah.