Diamond and Pearl makes pokemon get happy fast as hell. If you want to raise a Ralts, you gotta put up with GROWL for the first 10 levels. Ralts has absolutely no strength until it becomes Kirlia. Kirlia is a step up, but it's still not strong enough to take shit that's its level. Once you get to Gardevoir, you've got a psychic pokemon that can clear every gym for you.
Eevee; Go give it a massage, walk in that park, and battle once or twice without fainting. BAM, you get Espeon if it's day time.
Fighting with a pokemon that can't do shit til 30 and raising a pokemon without any real work. BIG difference.