We already know why Xbox and any other console was skipped over. The main reason why it sold less is because it was rushed. An unfinished product. The game felt incomplete. It wasn't until a year and a half ago or so that SFV started looking like the game it should have been in the first place. I'm sure had it released with the Arcade Edition content, SFV would have did the numbers it was suppose to. You're right though. It Capcom missed out on sales omitting the other platforms.
Also I can confirm that you don't need a separate disc for Champion Edition. I have it now and I've had vanilla SFV since launch. Capcom didn't lie when they said you would only ever need the base game.
You can't deny they lost sales by going exclusive for a series that was multiplatform since forever.
@bolded in my comment. I've addressed it.