Well, some Pokemon just take longer to evolve. Most pokemon that evolve to the 3rd evolution quickly, like most bug pokemon, have lower stats. These pokemon are good for ingame. Some pokemon evolve more slowly, but have more powerful final evolutions, like most Dragon and OU (overused) pokemon. Since they typically evolve in the high 40's (which requires a lot of training), they are preffered in competitive play, because, although they do take a while, they are the cream of the crop. Perhaps Black and White wants more pokemon to be in higher tiers. After all, there are many WiFi features in Black and White. that can encourage competitive play, so it would make sense.