It never really made sense. The Travellers Tales gameplay formula was only popular with kids but the add-on packs were (mostly) 80s/90s kids franchises with no relevance to their target audience. What 7 year old wants to play as Beetlejuice?
I've been wondering who was calling the shots on IPs for a while. It seemed like WB was really trying to keep it in-house with all the DC and HB stuff, so if they were keen on pushing the 80s properties maybe they're to blame.
The idea is that there would be content for the kids, as well as content for the parents buying/playing with their kids. You know, stuff they could teach their kids about and relate to them about. Also, I think the developers were just having a lot of fun with the stuff they liked.