The following is stuff I was told or overheard at San Diego Comic Con over the past several years and stuff I pieced together from things Kevin Kiniry and Jim Fletcher said officially, and so should be treated pretty much just as rumors/speculation. Some of this comes from Jim, some of this comes from Kevin, and some of this comes from retailers and toynews types that I chatted with at the Con:
Kevin and Jim were excited for spinoff lines and had plans to start STAS and Batman Beyond lines with individual figures just like the Batman line, but Superman and Lois were put together into one set, Terry, Bruce, and Ace were put together into one set, and the "Girls Night Out" five pack was created because comic shops at the time apparently wanted fewer SKUs to save on shelf space. These were inherently not ordered in large numbers, which isn't great for business, but the Superman and Lois and Batman Beyond packs still sold well enough that they probably could have kept going with STAS and BB lines. When the "Girls Night Out" pack sold poorly, due mostly to customers not wanting to buy a set with three figures they already own just to get two they didn't, that was the nail in the coffin for the spinoff lines, which apparently the bean counters higher up at Warner Bros Consumer Products weren't too keen on anyway, seeing STAS and BB as something that would compete with the Batman Animated line they were already starting to have trouble moving.
The JLU figures were apparently pushed from being retail bound to sold exclusively through DC Universe, gutted of accessories, and the end of that line as soon as it started, in part because of the above, and also due to someone at WBCP taking a more hands on role managing the business end of DCC after Kevin's departure, and deciding that branch of the company should not be investing as heavily as it was in action figures (especially unique sculpts) and should instead emphasize products that can be made cheaper like statues. I heard the only reason they were released at all was because they were already tooled, and Jim was adamant that they be made available to consumers in some way.