Why can't I trade my Swampert to X/Y?

I'm trying to trade a Swampert from ORAS to X and Y, but it won't trade. Its nature is Lax, it has Torrent as an Ability, and it is holding a Swampertite. Its attacks are Earthquake, Muddy Water, and Mud Bomb.

Solution 1:

Because Swampertite does not exist in X/Y, per Bulbapedia:

Pokémon holding Mega Stones introduced in Pokémon Omega Ruby and Alpha Sapphire (except Latiasite and Latiosite) cannot be traded to Pokémon X and Y.

This list of mega stones not available in X/Y includes: Altarianite, Audinite, Beedrillite, Cameruptite, Diancite, Galladite, Glalitite, Lopunnite, Metagrossite, Pidgeotite, Sablenite, Salamencite, Sceptilite, Sharpedonite, Slowbronite, Steelixite, Swampertite.

There are also other items that are present in OR/AS that would cause the pokemon holding them to be unable to be traded to X/Y; like the Primal Reversion orbs. These items and their effects were created long after the release of the first two generation 6 games.