The last new Ninjago sets were released in January 2013 and that was supposed to officially end the series. There’s a whole crapload of sets scheduled for 2015 but they didn’t make the January catalog.
Again I’m speculating here, but – the Ninjago TV series was a huge ratings success (my nephews loved it and I watched several episodes with them, even tho they were just a big ad for the sets); and the announcement that Ninjago will return after all came not all that long after the announcement that the Chima TV series ratings were a disappointment (I don’t know if it rebounded or not). Which seems like not a coincidence to me. Chima’s still having a long run though, at 2-2/3 years it’s as long as any series I’ve seen since really paying attention again in the early 2000s.
The other one they brought back is ‘Agents’. I thought the original 2008 Agents were OK, but I know from reading blogs n whatnot that a lot of fans consider it one Lego’s all-time best series, so that may have been a factor in creating Ultra-Agents.