Because the overriding sense throughout these two episodes is one of energy and fun. Even when danger seems imminent (such as throughout part two, with the impending arrival of Gavrok and his Bannermen), it's still an exciting sort of danger; you want to find out what will happen next, rather than dread what's coming. That's not to say it's a light and breezy story (the destruction of a bus filled with alien tourists is proof enough of that), but rather that they've got the balance between entertainment and menace right.
It was a good move, filming this at an actual holiday camp, because it gives everything a feel of verisimilitude as well as brightens things up no end. (Even if the camp does look a tad rundown in some shots -- but that's really what you want, isn't it?) Add in the surrounding countryside, which also looks gorgeous, and this is certainly a story that's nice to look at. And not only that, but it's filled with great actors as well. There are the big names, like Ken Dodd and Stubby Kaye (Stubby Kaye! In Doctor Who!), but all the supporting actors are wonderful as well, with Richard Davies as Burton, Hugh Lloyd as Goronwy, Sara Griffiths as Ray, Don Henderson as the amazingly evil Gavrok... everyone does a great job of making this story as entertaining and engaging as they can. Even Bonnie Langford seems to snap into focus, as Mel is asked to be more of an ordinary woman than a genius computer programmer with an eidetic memory (why Pip & Jane Baker ever thought perfect recall was an ideal characteristic for a companion is beyond me), and Langford seems far more interested (and interesting) here than in earlier stories. And while the Doctor has resorted to a few malapropisms, they're generally low in the mix while his other characteristics rise to the surface -- almost all of his interactions with Ray are rather wonderful to behold, giving off a sense of awkwardness mixed with genuine care and concern. McCoy is really coming into his own here, and the cliffhanger, where he's outraged at Gavrok's shooting at his white truce flag, is very well played.
It's a quick-moving story (well, only three episodes -- it has to be, right?), with some great dialogue ("Now, let me try and get this right. Now, are you telling me that you are not the Happy Hearts Holiday Club from Bolton, but instead are spacemen in fear of an attack from some other spacemen, and because of the danger, you want me to evacuate the entire camp?") and some fabulous performances. It doesn't feel forced or strained. So far Delta and the Bannermen is firing on all cylinders.