Is i because the Slogan *Gotta Catch 'em All!* has been dropped? Ash has caught significantly low amount of Pokemon since Johto because he's focusing on them as individuals! What better way to do this than have only 5/6 Captures....
A couple of things...
1. The catch phrase "Gotta Catch 'Em All" is a completely fabricated Western slogan made for audiences overseas. It has no bearing whatsoever on the original production. Thus, when it was phased out, nothing happened.
2. We aren't talking about the amount of Pokemon, but rather the
way they are captured. Specifically, we're comparing traditional capturing methods (like what you do when you face wild Pokemon in the games) versus the 'peaceful, make friends' approach that Ash institutes.
3. He's always treated them like individuals, from Day 1 (well, more like Day 2). They get shown respect whether he captures them like normal or invites them to come along.
Personally, I like the captures that are completely outside our expectations. Ash had to capture Snorlax to stop it from eating. He hatched Phanpy as an egg, but couldn't get it on his team right away. Chimchar was invited to tag along, but it had to leave Paul's team first. One of these days I'd like a Pokemon to casually walk up to a Poke Ball and let itself get caught completely out of the blue.