They chose to encode one bit per base (rather than two) to give them flexibility in encoding the binary values to avoid particularly tricky sequences for DNA sy

They chose to encode one bit per base (rather than two) to give them flexibility in encoding the binary values to avoid particularly tricky sequences for DNA synthesis or sequencing (such as ones that have many repeating bases, e.g. AAAAA, which are known as homopolymers, or ones that have high GC content).