I mean, after handily winning the playoffs, one would expect at least same money from merchandise in a higher league.
Like I said, we don't have the split figures from last season. For all you know your team sales did go up, but it might be your player's stats that made the difference, or all star appearances.
I don't know if all-star bonus differs by division, but if they're all the same, then 1 all-star starter should have 3000, so with 2 starters last season, you're already 6k worse off. I'm not sure how much a sub earns tho, apparently it's less. Take that away the 10k drop isn't too big actually.
In the end, without the breakdown of numbers, you won't know if there was actually a drop or raise due to promotion. But I would still say it's due to jersey sales, which actually makes up of the bulk of your merchandise but is affected by your player's numbers instead of promotion/relegation.