I personally believe that Darkrai is in the 10th movie because, honestly, THE TENTH MOVIE WAS ABOUT DARKRAI. It was probably planned to be so in the first place. New movies usually introduce an unobtainable legend spawning event potential, and they usually focus on the event legend involved. Dialga and Palkia were introduced because they ARE after all the cornerstone of Diamond and Pearl.
As for the inane ZOMG IT SHOULD HAVE BEEN GIRATINA! comment. First of all, Giratina is not officially confirmed to be a third part of a 'triangle'. They may have some similar features (like all being dragon type and having similar faces) but that's where the connection ends really. Let's have a look at the Sinnoh Creation Myth, shall we?
[size=8pt]In the beginning, there was only a churning turmoil of chaos.
At the heart of chaos, where all things became one, appeared an Egg.
Having tumbled from the vortex, the Egg gave rise to the Original One.
From itself, two beings the Original One did make.
Time started to spin.
Space began to expand.
From itself again, three living things the Original One did make.
The two beings wished, and from them, matter came to be.
The three living things wished, and from them, spirit came to be.
The world created, the Original One took to unyielding sleep...[/size]
So, out of no where, comes an egg, and out hatches the 'Original One' - obviously Arceus. Arceus supposedly first created Dialga and Palkia - creating the flow of time and expansion of space, and thus the physical world - and then life in form of Psychic Midget Patrol (Uxie, Mesprit and Azelf), who with their essence of Wisdom, Intelligence and Willpower - created sentient life. And then Arceus buggered off somewhere, leaving the world behind.
Typical creation myth (with some interesting parallels to Taoism - Creation started with a single energy, dividing into a duality, then into three parts, and finally into the rest of creation) rather similar to quite a few creation myths in our world. However, you may have noticed, perhaps, the conspicuous absence of anything even resembling Giratina?
There is no mention of Giratina anywhere. It's NOT part of some epic triangle. DEAL with it.
But then again, Giratina could have a different purpose. Giratina is described in the Pok