Complex sentence: The only dice used in the system are d2, d4, d6, d8, d10, and d12, because the authors decided to use two number increments and that the d20 wasn't logical for their purposes.
Split into simpler sentences: The only dice used in the system are d2, d4, d6, d8, d10, and d12. The authors decided to use two number increments and that the d20 wasn't logical for their purposes.