whywhywhywhywhywhywhywhy
We've been so long without a LUCARIO4ASH thread and then you had to go and ruin it. Dx
There have been so many Riolu and Lucario in the series already that for Ash to have one would be overexposure to the extreme. Lucario had its own movie, and was owned by Maylene and Riley, and Riolu had its own two-parter in the anime. As for dropping a team member, Ash didn't do that all the way through Hoenn, and he isn't likely to do that in Sinnoh. And besides, Ash has developed a close bond to all of his Pokémon - he isn't likely to release one easily, and he certainly won't drop one just to catch a Riolu/Lucario.
Well, unless he's an Auratard like everyone else in this bloody fandom, and he'll take one look at Lucario and immediately fanboy over it.