Seeing as they can't survive on sales to Amiga fans alone, the board is designed for use by industrial customers. They used an embedded PowerPC chip and the particular one chosen was probably the best chip for a reasonable price that they could get at the time. Amiga OS 4.x is compatible with PowerPC G3, G4 because thats what Amigaones have. However, those would require not just the G3/G4 chip, but a northbridge chip, southbridge chip, etc. Whereas embedded chips have all essential peripherals (minus graphics) onboard (e.g., USB, SATA, etc.).
Hans