Doug basically said he was bored of it and didn't want to do any more. he never usually does stuff for very long. i don't know about Lucy.
I suppose Primeval is unusual in SF series in that when people die, they stay dead, with the exception of Helen's bodyguard, called The Cleaner, and that's only because there's multiple clones. What is apparent, watching all the eps back to back, is that between seasons they decide to change direction, which is a bit sucky because things just get left dangling.with no real conclusion. Spotted an odd continuity error in ep401. When Connor's attacked by a large dinosaur, he runs, and for a split second it looks like he drops the stick he's carrying, but in the next shot it's still in his hand.
I dunno... to me, it seems pretty par for the course for British TV to give each season its own distinct storyline, resolve it, then start something new next season. And by the same token, annual cast changes have a fair amount of precedent as well. Look at Doctor Who. Even in the original series, it was unusual for a given combination of Doctor and companion(s) to last more than a year. And in the new series, there's been at least one change in the regular cast every season like clockwork. (To a lesser extent in the upcoming season, since the only change is Arthur Darvill going from recurring cast member to main-title regular.) Same with Torchwood. The Sarah Jane Adventures has kept basically the same regular cast for the past three of its four seasons, but only if you don't count K-9.
Emphasis mine. I do feel there were things dangling that didn't get a clear enough resolution. What Helen meant at Stephen's grave is the only one I can think of right now (I'm quite tired and have a job int tomorrow). She seemed to be implying something that wasn't followed on in the next series.
The thing that's confusing to me is that since that first series ended with the new universe there hasn't been anything similar when IMO they've done more significant 'changes' to the past in season 2 & beyond than they did in S1. In Season 1 they always made attempts to get the creatures back into the past and not disturb things. Once you added more military components as the show went along more creatures were killed and not returned to the past. Then you finally get to the biggest change to the past in the show's entire run. You had Connor & Abbey spend an entire year in the past killing fishies, stepping on bugs, etc... and they come back to an unaltered future? Obviously the arc has gone through changes, but that can be explained by it being the same timeline and just new management.
^As I said above, there's no remotely plausible way that any action taken 250 million years in the past could make such a subtle change to the present as a single individual having a different name and personality and a single organization being differently structured. A change that far back would've either had no effect at all or prevented humanity from ever evolving. So there's no point in expecting any kind of consistent logic here, since it never made sense in the first place. Maybe it wasn't Nick and Helen's actions in the past that caused a change in history. Maybe the anomaly they passed through simply had some kind of dimensional hiccup and deposited them in a slightly different timeline than the one they left, one that spontaneously arose on its own far more recently. Like if you travel a long distance down the highway, then come back and accidentally take the wrong exit close to home. After all, quantum physics says that alternate histories may be spontaneously emerging all the time. Indeed, theory suggests that, contrary to what we assume in fiction, time travel couldn't create an alternate history, because your quantum entanglement with your own timeline of origin compels you to take only those actions that would bring it about. So any alternate timeline wouldn't be something a time traveller caused; at most, it would be something that already existed and they found a way to visit. It could happen if there's some sort of nonlinear quantum process taking place in the anomalies. They could be passages between timelines as well as times.