The most important reason is the gameplay. I am a firm believer in that a game should introduce something that has never been done before. Valkyria Chronicles delivers. It laid the template that all other SRPG developers need to follow. It took the traditional SRPG with its square and hexagonal chess-like playing board, and turned it into a completely 3-D experience! Instead of moving on a square grid you select your unit and are thrown into the 3-D watercolor world in third person, and can walk around and explore the battlefield. The system has you selecting your unit, and heading towards your destination. Dodging bullets, tank fire, and finding cover is part of the journey. But the really interesting part is that when you go to fire, the action is paused. So you can aim, and characters statistics for accuracy are taken into account. This allows for perfect head shots.
Valkyria Chronicles is just one of those titles that can pull people who never played the genre in and make them fans. I highly recommend it. This is one you need to pick up now while it can still be easily found at your local used game store, because this will easily be one of the most sought after PS3 games a few years down the road.