Why not everything?

In a recent post, I wrote about balanced audio cables and how they work. Unlike simple RCA cables with their single signal conductor and ground wire, a balanced cable has two signal conductors with opposite polarity signals. The receiving equipment amplifies only the difference between the two opposing signals and ignores anything in common, such as noise or distortion. Balanced cables are by far the better choice when connecting audio products together. And if that's true why do most audio products continue with only single-ended RCA for their interconnection options? Why would manufacturers such as PS Audio, concerned to the point of (sometimes) lunacy about sound quality offer a lesser means of interconnecting equipment? Old habits die hard. The need for compatibility often trumps our best intentions. I am going to wager that 80% of high-end audio systems still rely upon their RCA connectors for their goes-into and goes-out-of. My guess is that if you asked a cable manufacturer their ratio of sales between XLR and RCA cables that my 80% guestimate would be pretty close to right. With as much attention to tweaks and perfection in equipment that we as a group lavish upon our systems, you'd think that percentage would be flipped around. But I suspect you'd be wrong. Old habits die hard. Even the ones so clearly wrong.