Sequels, they're fraught with difficulty. What do you do? More of the same and people get bored; revolutionise and you alienate the reason people liked it in the first place.
A short run down of the sequels that worked (from no medium in particular):
Alien and Aliens
Mass Effect
Star Wars Trilogy (4-6)
A shorter run down of those that don't:
Police Academy 2 onwards.
Alien 3 and Resurrection
Neither arbitrary grouping follows either set - some give more of the same (mass effect and Police Academy), others revolutionise (Aliens and Alien 3/Res). Being very different helps (Aliens), but being part of the "bigger picture" makes the key difference. Each episode builds on the last, offering the player/viewer something new but reassuringly familiar.
Of course, I'm missing one from those that don't: SupCom 2. The original SupCom wasn't perfect - you could be utterly reckless with your armies in pursuit of victory and the game itself would bring most hardware to its knees, even the plot was quite thin ("We want to beat the others to a pulp, let's rock!"). But it was enough for the "fate of the galaxy" to be in your hands with a vast army of robots crossing enormous distances on a strategic scale. Despite its flaws, it worked.
SupCom 2 doesn't. It has tried to go a bit Starcraft with the personal plotlines, but you're still ultimately chewing through a large army of soulless mechs. And nothing makes me want to care about my Commander. The maps aren't that large and aren't particularly "realistic" compared to the original. In theory it follows on from SupCom, but somehow fails to capture what the first achieved.
So to sum up, the two games are like a wine glass - SupCom was a fine drinking vessel, something you kept clean and used to entertain guests. SupCom 2 is the same glass...after you've dropped it on the floor. All the pieces are there somewhere, but it's never going to be usable again.