Interesting question! I'm not an expert, but I suppose the answer is convenience of implementation? By using Bitcoin, the system is already in place for miners

Interesting question! I'm not an expert, but I suppose the answer is convenience of implementation? By using Bitcoin, the system is already in place for miners to verify signatures, tokens (i.e. "civiccoins") to pay for getting stuff verified on the chain by the signers, a well made and proven decentralized protocol, etc.