To get them to spend money on all the 20th Anniversary stuff coming out? I mean if the video I saw was the exact ad shown in the Super Bowl, it had Pokemon20.com there to get people to hopefully check it out. Before people have pointed out that outside of Lucario, the ad relied heavily on Generation 1 Pokemon probably in attempt to appeal to those who fell off the Pokemon wagon shortly after Red/Blue/Yellow.
I thought it was pretty jaw dropping, they purposely tried to make an ad to appeal to the masses and encourage all trainers out there by saying "Hey world! You wanna be the best Pokemon battler? You can do that." Secondly Nintendo's grubby fingers probably weren't terribly involved in this, I suspect it was mostly a Game Freak/TPCi thing. They expect money blown on the ad to come back in sales made by 20th Anniversary purchases and Pokemon literally being a money printer.