The easy answer, of course, is that the Pokémon movies still make money. The longer answer is slightly more involved. Since 1998, there has been a new Pocket Monsters anime in Japan every year, like clockwork. And not only are they released each year, but they are always released during the same month: July.
There is a very specific reason for this: the vast majority of Japanese school children are on summer holidays in July, and are, in turn, looking for something to do. Likewise, parents might be looking for something to do with their kids and going to the movies together fits that bill. Summer holidays in the northern hemisphere continue through August, but that month is occupied by the 0bon holidays, making July the prime month for kiddy entertainment in the local cineplex.
Pocket Monsters isn't the only franchise attempting to take advantage of grade schooler's holiday time. Doraemon, an earless robotic cat from the future, typically gets a new movie every March, during school's spring break. Other anime, such as those from Studio Ghibli, are sporadic or, in the case of Western animation, dependent on localisation or distributors. But anime like Pokémon can be released yearly, like clockwork.
In the West, the Pocket Monsters movies are released in various months, whether it be June or January, February or October. It's not consistent. And kids tend to like things that are steady and constant. That's exactly what they are in Japan, which is, in an odd way, comforting.
This year, Pocket Monsters are doing one better and releasing two movies this July: Pocket Monsters Best Wishes! The Movie: Victini and the Black Hero: Zekrom as well as Pocket Monsters Best Wishes! The Movie: Victini and the White Hero: Reshiram. These are two full-length motion pictures, and not shorts. The movies will feature different legendary Pokémon, and will be shown one after another in movie theatres, like a double feature. This is a first for the films, and no doubt many kids will stay on for the second picture.
Japanese children often go through phases. When they are small, boys might play with Tomica (Japan's version of Hot Wheels) and then move on to Ultraman or Kamen Rider. And then perhaps, Dragon Ball. As in the West, toys are often clearly divided between the sexes. Pokémon, however, is far more fluid and able to appeal to both young boys and young girls as well as children of a wide range of ages with small children enjoying the anime and cute characters. In that regards, it is somewhat unique. Recently, Pokémon card sets for boys and sets for girls were released (it's worth noting that only the pink version says "for girl", and the black version does not specify). Only the packaging is different, and the card game itself and its mechanics are the same.
Like with many kid franchises, however, children grow up and move on. They become less interested in Pokémon, and more interested in other things - sports, the opposite sex, music, whatever. But for those few years, Pokémon is able to appeal to a wide variety of kids with its anime and video games. Pokémon is able to, well, catch 'em all.