O course everyone is encouraged to come. Why would anyone think that only "yeshivish" rabbanim and sects are being encouraged. Who actually attends is a completely different story...
Of course e/o is encouraged. But if you want true unity, then you should so your best to invite representatives from all the communities with whom unity is desired.
They probably didn't. The message at the Asifa will include: Don't watch TV shows online, don't visit sites like Facebook/twitter/pinterest/forums/blogs/CNN/Vosisneias/youtube etc etc.
MO (and also many mainstream orthodox - thus the probable low turnout) has no interest in hearing that.
Exactly, most mainstream Orthodox don't want to hear it, but they're still making an attempt to get the message across. So show 'unity' and invite the rabannim of other ppl who also don't want to hear it.
I would imagine that every religious leader, no matter their stripe, understands and recognizes the danger of the internet, and the countless families its destroyed.