Much more likely they had a strict time limit (internally or externally imposed) on what they considered biodegradable. If you make it stronger but thinner, bacteria can break it up much faster which leads to faster breakdown. By comparison the bag being loud seems pretty irrelevant if it's just not possible to make a thicker bag degrade faster.
Also, many biodegradable plastics are more brittle than more common plastics. They only way to keep the bag flexible in that case is to make it thinner.
Also, many biodegradable plastics are more brittle than more common plastics. They only way to keep the bag flexible in that case is to make it thinner.