Trouble is back in the 1970s films weren't often conceived with the idea of sequels. Now, it's part of the thought from the very start. I think it's possible that some film makers are envious of the 16 ep seasons that some TV dramas get and want to spread a story across more than a two hour film, the problem is the time it takes to make a film, and the fact that it's box office draw has a direct effect on it's budget makes it a more precarious way of telling a long story.
But in a way I think it's ok, if you're incredibly patient.