Can K-type thermocouple disconnects be used with thermocouple based 3D printers?
Not sure if this is the reason, or just A reason, but thermocouples create a pretty small voltage for a given temperature, whereas thermistors have a larger (easier to read) change in resistance. I would expect that a thermocouple would require slightly more sensitive instrumentation to read it than a thermocouple. And seeing as space requirements aren't that significant on a 3D printer using the easier to read thermistor, despite its size, seems like the better option.

I prefer thermocouples myself, since they don't need calibration and effectively lifetime guaranteed (aside from mechanical failure). However almost every project I've ever worked on, the Sparkies always seem to push for thermistors rather than thermocouples.