Without knowing what was going through FFG's planning of both Star Wars and X-Wing, I could offer a couple of possibilities.
For X-Wing, there just wasn't a place for the B-Wing, yet, in the lineup. Wave 1 was about launching the game with its most iconic imagery and namesake; the X-Wing fighter. On top of that, you're naturally going to include the most recognizable X-Wing pilot of them all; Luke Skywalker. Naturally, the opponent of the X-Wing is the TIE Fighter, so there's your Core Set.
The next step is to grab the most recognizable Imperial "pilot" from the movies, which leads to Darth Vader and the TIE Advanced. At that point, in the interest of no doubt tying everything back to the first movie, the Y-Wing was the natural second ship choice for the Rebels.
So now we're on to Wave 2. FFG tosses in the Millenium Falcon and Slave I, adding a new dynamic to the game with the large ship base, and providing for one more ship on each side. I would certainly argue the TIE Interceptor as the next most popular option to run with on the Imperial side, and the same with the A-Wing. In fact, Wave 2 looks like a nice balance between the two forces, as the craft are comparable to each other.
The feeling is that Wave 3 will have the TIE Bomber and the B-Wing as no-brainers, so expect to see it there. It's not like it was overlooked or forgotten, you just need to accept that business comes first, and they no doubt ascertained that the B-Wing won't be moving units like a Millenium Falcon or a TIE Advanced would. By the time the B-Wing comes out, if a potential new player saw it and said "meh", at least the rest of the model range is there to entice them.
On the card game front, even though we got a lot of stuff in that Core Set, we really didn't, at the same time. What I mean by that is these objective sets need to have not only their 5 Command Deck cards make sense as a group, but also synergize on some level with other objective sets in the other affiliations so as to make for some level of deck buidling. It may not be much, but then again, it's the first of many sets of cards, so we shouldn't be expecting "everything" to be in there.
I was surprised to see Ackbar and Home One, but at the same time, I wasn't disappointed that there was no Executor or Millenium Falcon. They'll all come in time, as the game expands, and I like to look at that as having something to look forward to. If the B-Wing is your favourite thing about Star Wars, imagine how happy you'll be when you see that preview article on the news page that announces it in an upcoming Force Pack.