When they started to add better physics/effects and voice them like bigger budget games they made their job that much harder. Plus open worlds.
Not sure why they did all that to be honest. I was a big fan of the more nuanced, linear games lol
They still pumped them out fairly regularly even with voice acting and the open world hubs. They released at least 2 or even 3 a year up until 2018.
The reason this one is taking longer is that they're actually changing up the structure quite a bit (open world universe with planets you can explore at will across different time periods, at least I think thats what they said from previews).
It's also going to have 9 movies of content at minimum (it'll probably also have Solo, Rogue One, and who knows, maybe even TV Show stuff like Mandalorian and Clone Wars), which is more than they've ever had in a single game before. Maybe some of that will be DLC later I wouldn't be surprised.
The devs taking their time with this one is a really REALLY good sign IMO. The Star Wars ones were always my favorite, so if they can take all the time they need.
Still hope it makes it out this year, but if LEGO is already listing it as 2021, then it's probably not likely.