Pokemon is recognized for being a bestselling and popular long-running game-to-anime franchise. Yu-Gi-Oh! is similar, but focuses more on card games. This is also probably the reason why Danball Senki (Little Battlers Experience) is also included, because it showed the aspect of anime that is closely linked with gaming and gaming merchandise.
K-ON is very popular with women as well, and is probably one of the most popular stories of anime tourism. Toyosato Elementary School might have been demolished if not for K-ON!'s use of the school as basis for Sakuragaoka High School.
Strike Witches, despite its immense amounts of fanservice, gave a unique twist to alternate history by depicting a version of the Second World War as cute girls fighting aliens.
Well, despite my thoughts about this, I'm sure people will still push that other series deserve the slots better than those in the list, but I'm sure that there's a reason why these series are recognized in the Fifty.