First off, it's "coaxial" cable, not "RCA" cable.
What I think you're referring to is coaxial cable with RCA plugs on the ends. Coaxial cable comes with otgher types of jacks on the ends besides that.
Second, and more important, coaxial cable is shielded, which means it rejects external interference from external sources of electrical noise.
If your application isn't prone to electrical noise, then I suppose parallel, unshielded speaker cable would work but, the lower the signal level, the longer the run, and the more gain applied upstream, the more susceptible to noise it becomes.
Speaker cable is passing a high level signal that require additional amplification so shielding is not really needed.