Why does the auto brake panel on an A320 use "lo", rather than "low"?
Boeing 747 wiper knob showing LO; source: alamy.com


As shown above, it's not unique to Airbus, or to just airliners, so from that perspective:

Keeping the same standard as on-screen messages$^1$

Example A320 ECAM messages:


  
  CAB PR LO DIFF PR
  ELEC IDG 1 OIL LO PR
  FUEL CTR TK PUMP 1 LO PR
  FUEL L WING TK LO LVL
  HYD B RSVR LO LVL
  


Note: Messages on displays are abbreviated to save space, and are in all caps for readability (see: Why do aircraft cockpit displays use uppercase fonts?).

It wouldn't make sense to deviate just for the auto brake (its ECAM indication is also LO).

That labeling is also typically electroluminescent, so the fewer the letters the cheaper (not just for LO, not just for airliners).



$^1$: FAA AC 20-175 - Controls for Flight Deck Systems


  Labels are the most common means used to identify and describe controls and other devices in the flight deck. They can be full text (e.g., "Standby"), abbreviated text (e.g., "STBY"), acronyms (e.g., "AGL" for "Above Ground Level"), as well as icons (e.g., [icon example] for "On/Off").
  
  Control labels must be visible, legible, and understandable for the population of pilots that will use the controls, per § 2X.1555(a).
  
  Use only one abbreviation and/or one icon for labeling a function. This is to prevent confusion when a label appears in multiple locations. [emphasis added]


The AC also references SAE standard ARP 4105B Abbreviations, Acronyms, and Terms for Use on the Flight Deck (paywall), and ICAO Doc 8400.