Why was the word "ربما" (rubama) mentioned with a softened ba (ب) and not a sharpened one in the verse, "Perhaps those who disbelieved will wish that they had been Muslims" (15:2) from Surah Al-Hijr?

The verse: (Perhaps those who disbelieved will wish that they had been Muslims) [Al-Hijr/2] is an account of the disbelievers' regret over their disbelief and their wish that they had been Muslims, either at the time of their death throes, or when they are presented to the Fire, or on the Day of Resurrection when Allah will bring out the disobedient Muslims from the Fire, and the disbelievers will wish they had been Muslims.

Nafi' and Asim recited "rubamā" (perhaps) with a lightened bā', while the others recited it with a strengthened bā'. Both are eloquent and widely transmitted linguistic forms. The word "rubb" (often translated as "perhaps" or "how many a") can denote both rarity and abundance. The Arabs used it in both contexts. The intended meaning here is that the disbelievers will wish on many occasions that they had been Muslims.