Dialga is known for sporting defensive sets due to its typing and access to Stealth Rock. You might see the uncommon Choiced set here and there, but generally you will not see Adamant Orb. Dialga finds more utility from Leftovers or something like a Shuca Berry, or raw power from Specs or Life Orb. Dialga isn't as fast as most offensive Pokemon in Ubers, leaning it to become more Defensive. Dragon / Steel isn't as great of a offensive combination due to Primal Groudon being absolutely everywhere in the metagame, meaning Leftovers / Specs / Scarf / Life Orb pretty much outclass it.
Palkia uses Lustrous Orb only because its known for running Choiced sets, and its a good bluff item (like Expert Belt on commonly Choiced OU Pokemon, like Keldeo). Also, Palkia is faster, and hits the 100 Base Speed tier, which outspeeds a surprising number of Uber Pokemon.
Giratina obviously uses it to become its Origin form.
