Orange on black is pretty easy to read so that's my guess but I've no idea for sure.

Red LED's were the most common in the 70s when this stuff popped up, but the LED tech then was terrible - Gameplan used them in their displays and they never looked very good IMO.

Gas Plasma displays were likely a huge leap forward at the time - other contemporary display tech at the time was nixie tubes (expensive, still needs HV supply), vacuum fluorescent (as seen on Gottlieb displays), LCDs, and LEDs.

Also, they were incredibly cheap - the tubes were less than $5 at the time (unlike today!)

It’s a high voltage gas display. I believe that’s the color due to original specs used in 1989-1990 when they were introduced in pins originally. These were carryovers from alphanumeric displays used.

Article here: https://books.google.com/books?id=ARiQpmsT9BYC&pg=PA92&lpg=PA92&dq=pinball+display+color+1990+why+orange&source=bl&ots=7U7r6fBmUF&sig=ACfU3U10VKeibgLC9QW2VaLCOTB2xmkkHw&hl=en&sa=X&ved=2ahUKEwiszeKFxe_oAhWfgnIEHeSqC2sQ6AEwAHoECAoQAQ

my guess is that it's just the plasma that was used at the time. They're just a updated brethren of alphanumeric displays, those are plasmas too and orange. under the history tab it shows a black and orange plasma display screen https://en.m.wikipedia.org/wiki/Plasma_display

And earlier, like the 6 and 7 digit early Stern/Bally displays.

They’re neon filled. Neon is orange when excited. It’s excitation voltage is approx 65V.

Yeah had an argument with some faux pinball collectors about this. Neon is a noble gas that will emit light that is orange when excited with an electrical current.
Other colors you see are from different gasses.

This website has some great information: https://sciencing.com/neon-its-colors-4927221.html

So most likely it’s because sourcing a ton of more expensive gasses sucks and neon was commonly used and abundant.