Being able to see it in theaters is due to the efforts of GKIDS, the distributor for some anime in the US. Last year, GKIDS sponsored Studio Ghibli Fest, which put one Studio Ghibli animated movie in theaters each month. It was apparently pretty successful, so they are doing it again this year. Studio Ghibli is Hayao Miyazaki's studio, the most famous Japanese anime studio. It created films like Totoro, Castle in the Sky, Princess Mononoke, and Spirited Away.
If you are interested, you can see if any theaters near you are playing it by going to the Studio Ghibli Fest website, scrolling to Porco Rosso, and putting in your zip code.