I know its a rumour but:
I heard once that reason why such great and quite modern army (compared to many models from other ranges) as Tomb Kings disappeared was very sad mistake. Rumour has it, that moulds for Tomb Kings range has been accidentally destroyed. If it is true, I don't think they will made new moulds for "old looking" range because it is more profitable make new moulds for new models. More people will buy it. And as all we already know high quality moulds for plastic ranges are very expensive.
I was going to start new TK army just when AoS came out and then TK's disappeared. I was very disappointed, so I'm looking forward to see new dethrattle army. I hope it will have nice and interesting look, and will combine somehow with NH and LoN.
(...but as an archaeologist I will always miss TK's. If I could afford old second-hand models I would buy them, but today's prices are out of my range)