Why so few balanced phono inputs?

2 things:
If you choose to replace an RCA male plug with an XLR, the convention is that the hot wire into the RCA plug should go to pin2 of the XLR. The "other" signal-carrying wire goes to pin3. Ground and only ground attaches to pin1. But as John Tracy intimated, the shield of a typical RCA-terminated single-ended cable, if used to connect to pin3 (carrying the negative phase), may cause issues in sonic quality. Which I suspect has something to do with why many say that balanced is inferior to SE. If you choose to connect the cartridge in balanced mode, best to have two conductors of equal quality for the plus and minus phases of the signal. It's not hard to make such a cable for yourself.
Maybe that was only one thing, not two.

I agree that balanced connection is not per se necessary for superb sound reproduction. But where do you get the "110 ohm vs 75 ohm" idea? The impedance in a typical audio system will be only be that which is reflected by the interconnect, depending upon the output impedance of the driving device and the input impedance of the driven device, no matter how you slice it. Further, I thought "pro audio", where distances are great at times, use 600 ohms.

Further, it has been my experience that those manufacturers who offer true balanced connections are prone to say why it is superior to SE connections. Conversely, those who make only SE devices (like Manley Labs) are prone to diss the notion of balanced connections. So, I would take Ms Manley's opinion with more than a grain of salt. If done well and properly, there is no doubt in my mind that there is an advantage in noise issues associated with balanced connections, especially from cartridge to phono stage, if only because of fewer problems with grounding. That's just a "feeling" based on 3 decades of experience with both types.