X and Y are not directly compatible with Gen III. To get a Pokemon from Gen III to Gen VI is a pain in the ass. You need to find a Gen IV game and either DS Phat or DS Lite, then Pal Park it. Then, you need to find a second DS (or 3DS) with a Gen V game and Poke Transfer it. Then, you need a 3DS with Pokemon Transporter on it to move it to the Bank, and then finally to X and Y from there.
That's quite a few steps and, at least for Gen III - Gen IV, requires hardware that many people simply don't have anymore (or at least, functioning). Plus, if you're like me, I restart my games a lot and rarely have completed saves. So, if I wanted to do this, I'd have to play through potentially several games just to do the transfers.
And that's just for one Pokemon. Can you imagine if you had a PC-full?
I had a Living Pokedex on my Pearl version, so that already started on Gen IV. To transfer the 490-some Pokemon all the way to Pokemon Bank took several hours.