Here's the reasons I think the series is over and done: (Some are silly but whatever.)
1. Too many characters to keep bringing over to the new game, maybe comprising time for new content. Also Activision wouldn't want to risk money for extra development time with sales dropping and franchise becoming unstable.
2. The Bala brothers left mid way half a year through the supposed development for new skylanders game they'd be releasing this year (Maybe Activision gave them the 'nod' that things weren't looking good. The rest of the team probably weren't told yet or they'd been busy on Crash)
3. Stock in stores this year are really low compared to last year when the shelves were overflowing. (Same happened just before Infinity's cancellation and if Infinity went down it wouldn't be long till the same happened with Skylanders.)
4. The recent price plunges for Skylanders SuperChargers, seems stores are clearing all out. (While they can before Official word comes out which'll leave bad taste with consumers that it's not worth investing into a cancelled franchise.)
5. The logo color this year is all dark, they started with full white/slight blue so maybe this is the finishing logo. (silly but true)
6. The gimmick this year, creating our own characters and even getting a figure of it! That means new characters would be more difficult to create. It also seems like this would be something kept for the last game. Also if a next game came about, would these be compatible, if not what could they possibly do to innovate from this and everything else we've had thus far.
7. No updates to the game since the launch one. (Hmmm)
8. At Vicarious Visions, aren't they working on Crash? Surely it'd be too much to do Skylanders too.
9. The poor performance of the Skylanders Academy series.
. The plot also seems stuck with Kaos, Kaos from the beginning, Kaos till the End! They also recycled Racing and no pvp or other game modes this year.
10. Activision is all about the money... (I know, long time COD fanboy here
)