Why one meal a day?

[SecretSage]

Many times it says

“He eats only in one part of the day, refraining from food at night and from eating at improper times." - DN 2

I'm not sure the exact reason for this rule.

It could be to promote health or promote self-discipline, or both.

It says in MN 21:

“Once, I addressed them: ‘I eat my food in one sitting per day. Doing so, I find that I’m healthy and well, nimble, strong, and living comfortably. You too should eat your food in one sitting per day. Doing so, you’ll find that you’re healthy and well, nimble, strong, and living comfortably.’ I didn’t have to keep on instructing those mendicants; I just had to prompt their mindfulness." - MN 21

Seems to imply that it's for improving health, strength, comfort.

Then it lists as one of the five factors of meditation (for quickly achieving arahantship):

(2) “He is seldom ill or afflicted, possessing an even digestion that is neither too cool nor too hot but moderate and suitable for striving." - AN 10.11

It also lists one of the five unfavorable occasions for striving:

(2) “Again, a bhikkhu is ill, overcome by illness. This is the second unfavorable occasion for striving." - AN 5.54

So developing good health is an essential aspect making achieving arahantship here and now easier.