There's a 16 month period between the Fighter's Pass announcement and its expected finishing deadline. Assuming the last one is announced months before its release, which could be as late as the deadline month, we're essentially half way through the period and we've still only got one character known. And E3 is prime for such big announcements, especially if they're announcing some of the crazier ones first (maybe like the base game, they'd generate the most interest by showing off the most impressive ones first?). Plus there was that note from Skaurai saying they worked on fighter 2 and 3 alongside Joker, apparently.
Sure they could space out the reveals, but they could also actually give us a good idea of the Fighter's Pass content by now? Plus they may not be releasing at the same time anyway, so they'll still get their focus as one releases months after the other.
I just wonder if there'd be space to do two trailers if it's truly just the Nintendo direct at E3 this year. Would we get the two trailers without Sakurai's DIRECT commentary and other stuff like more Mii costumes? Only time will tell I suppose.