It's a pretty restrictive design space. The Stratocaster shape is based on the centuries-old shape of acoustic guitars. The lower dip of the "hip" of the guitar shape serves a practical purpose, in allowing it to rest on your leg when playing seated. The upper dip mirrors the lower hip aesthetically and removes weight. The cutaways in the upper bout allows you to access the higher frets. At that point there's not a whole lot of design variation left without sacrificing some functionality (eg the Flying V shape is not really playable while seated), and honestly most of that design space has already been explored, too. The strat shape is so common because, well, it's a really good and obvious design for an electric guitar, which is why it was one of the very first. If you stray very far from one of the handful of established designs, you pretty much just end up with a worse guitar.