Joe,
A number of reasons.. In no particular order..
1. Lack of transistors.
Invented in 1947, it's unlikely that transistors were used in the recording of the Jazz records from the 1950's. All equipment would of been valve based and probably hand-wired, point-to-point as well with PCB technology still in its infancy.
2. Single-mic technique.
Rather than running mics for each musician through a mixing desk, a favoured technique for Jazz recordings was to place a single mic in the room and then move the musicians backwards and forwards from it to get the correct balance.
3. Tape.
By recording one mic to a mono tape you can effectively record, mix and master simultaneously. I imagine the first play of the tape was to check the recording and make a copy, the second to press the record. As tape degrades with each play you can't get much better than this.
4. The power grid.
Very much like the roads in the 1950's.. the power grid was effectively deserted. Hardly any TV's, no switch-mode power supplies, very few devices on the system creating interference meaning very little distortion feeding back on the AC power wave.
5. 15A round pin plugs / bulgin type connectors and no fuses.
A slight but noticeable difference compared to today's 13A square plugs and IEC leads.
6. RFI / EMI and other interference.
Negligible compared to todays saturated airwaves.
7. Copious amounts of drugs.
Always makes music sound better.. period.