I don't think Dexter was ever meant to be a "cross-every-t" type of story in the first place (more graphic novel-ish), but IMO the answer to these questions is because the quality of writing/producing on this show has declined massively/the people running this show do not think they have a smart audience capable of remembering details from early (awesome) seasons. Or are trying to cater to a very broad audience and not people who are paying attention to details not explicated on "previously on Dexter!". Or are going to surprise us all with an amazing upcoming season and all will be forgiven. Sorry... but this used to be my favorite show, and the Deb thing at the end of last season did help I'll admit, but the past 2 yrs have been such crap that if it gets 3 eps into the next season and its more of the same then I am going to have to officially say goodbye to this show as a quite a few other TWoPers did last season.
I agree completely that the show is one that requires an amount of suspension of disbelief to enjoy (its a show about an affable mass murderer after all) but its just gotten more and more outlandish. To highlight the contrast: in the first season finale, we're suppose to believe that Dexter took Brian back to his apartment and that there were no cops posted to gaurd this wanted killer's lair in case he was to come back. That's a...stretch but it's more of a fridge logic type of stretch, something that you don't think about until the credits roll.
Now compare that to the massive plot holes in the previous two seasons. Dexter calling 911 and not disguising his voice; no one persuing the fact that Jordan Chase just kinda mysteriously vanished or that there was a crashed car found right by the camp where they committed the crimes( with hair and DNA all over it); Dexter [i]sending a video message to the Colin Hanks who's a wanted man[i. This kind of shit is just too ridiculous to buy that rips you our of the story
Sigh. Maybe I've just been spoiled by shows like Breaking Bad that go to great lengths to keep a tight consistent story. But I am defintely on board the If-the-season-7-premier-doesn't-rock-I-am-gone bandwagon.