Why do most phono preamps lack XLR input even thought cartridges are naturally balanced?

@atmasphere  thanks for your clarifications. I am using an Ayre P-5xe which has balanced phono inputs.  I was bothered that if one uses the rca inputs to customize resistor loading ( I do not) that the use of one resistor per channel might indicate it was not truly a balanced input. Your explanation of the differential connection clarifies that worry.I have wired my own balanced DIN to XLR phono cables (refusing to pay $600+ for something so simple). Of course pins 2 & 3 are signal, and pin 1 is shield on the xlr ends only. I have a drain wire to the center DIN pin as a drain  connecting to the P-5 ground. Is this correct? I have no hum.

 To clarify, I can only add the specs for my Ayre P-5xe: the gain for the balanced output is 6 dB higher than single ended out for each of the 3 gain selections. However the gain does NOT change with the choice of RCA or XLR inputs. You should be able to look up the specs and recommended phono cable wirings on the Ayre website in the manual download.  The specs are minimal and no S:N specs are given.