I am probably wrong here, but it looks like Wave 7 gives Scum 3 ships, Rebels 1, and Empire 1.
My theory is FF went left field on the two ships for Rebel/Empire to give some people a reason to buy them. Alot of Empire/Rebel players all ready have their builds set and needed to be encouraged with what could be OP ships for $29.99+.
I don't know too much about ships and lore so feel free to bust my balls.
I am probably wrong here, but it looks like Wave 7 gives Scum 3 ships, Rebels 1, and Empire 1.
My theory is FF went left field on the two ships for Rebel/Empire to give some people a reason to buy them. Alot of Empire/Rebel players all ready have their builds set and needed to be encouraged with what could be OP ships for $29.99+.
I don't know too much about ships and lore so feel free to bust my balls.
What's the third Scum ship?
Okay, explanation:
Scum is behind on ships, so this is FFG catching them up. The Hound's Tooth is Bossk's ship, and Alex Davy seems pretty keen on getting all the bounty hunters (or at least the practical ones) into the game. With Bossk, we now have the three best looking ESB bounty hunter ships: the IG-2000, Slave 1 and Hound's Tooth.
The Kyrax (I'm not even going to try and spell it properly) is a Scum X-wing, it fills a niche they didn't have.
The TIE punisher and the K-wing are what the community's started calling "super bombers", expensive ordnance carriers who look like they'll fight primarily with ordnance. We don't know how they work yet, but ordnance has been very weak in the game for the first six waves and Wave 7 is clearly FFG's attempt to fix it. We've seen no cards yet, so we don't know how they'll fix it. This is most of the reason why we have these two ships in lieu of more "fan favourite" ones: the other ships may be arguably better looking or better known but they're not super bombers. These two ships were chosen for what they do, not how they look.
The 3rd looks like that black Star on the left. Looking in the background we see 3 or 4 other ships but they are not as easy to see or read.
These two ships were chosen for what they do, not how they look.
In terms of its role as an ordnance platform, the XG1 does more or less (but not exactly) the same job as the Interdunisher.
So FFG could have chosen either.
It could be argued that they might have chosen the Punidictor because its a TIE variant and therefore much more instantly recognisable as an Imperial ship.
The 3rd looks like that black Star on the left. Looking in the background we see 3 or 4 other ships but they are not as easy to see or read.
Scum get two ships. Empire and rebels one each.
The 3rd looks like that black Star on the left. Looking in the background we see 3 or 4 other ships but they are not as easy to see or read.
I assume the that black on the left you are referring to is the Starviper which is already released.
We know what the 4 ships are from the presentation. Two scum, one rebel, one imperial.
Wave 7 is the ordnance fix wave. That's why we have to ordnance carriers for Rebels and Imps.
If the Rebels and Imperials are getting SBombs, where is the Scum one?
You have to buy the Reb/Imp blisters to get them! 
They will probably put a missile fix in with the Rebels and a torpedo fix in with the Imperials.
@OP title
1. because new ships keep the game alive
2. Star Wars space ships are cool
If the Rebels and Imperials are getting SBombs, where is the Scum one?
Andrasta Title on a Firespray. Tadaaaa! Superbomber.
If the Rebels and Imperials are getting SBombs, where is the Scum one?
I don't really think scum need superbombers. They strike me as not as millitry minded as most factions, with the most expensive ships belonging to bounty hunters (most of which can carry both bombs and missles and proton torpedos, not currently withcounting Bossk who is unknown) that can serve the role given a pinch. Most ships strike me as low budget vessels that are retrofitted up to be attack vessels or cannon fodder, not quite up to either empire or rebellion specs.