Why does the Quran omit the letter "ya" from the end of some words, such as "yasri" (يسرِ), "qawmi" (قومِ), and "rabbi" (ربِ)?

The letter yāʾ was omitted in the Almighty's saying: (and the night when it departs) in accordance with the Uthmanic script of the Qur'an, in consideration of the ends of the verses, and sufficing with the kasra before it. Similarly, the yāʾ was omitted from (O my people) and (my Lord) for brevity and sufficing with the kasra before it; for when the diacritics convey the meaning of the letter, especially in calls and supplications, the diacritic suffices and the letter is omitted, as calling (nida') is a domain of change and omission.