The alphabet as it stands here has four yus letters: a front nasal and a back nasal, each with a plain and an iotified form. That is a complete and symmetrical set, and it lasted almost nowhere.
The reduction happened in stages and for different reasons in different places. The nasal vowels themselves were lost early in the East Slavic and most of the South Slavic area, which removed the reason for having any of the four. Where they survived longest, the surviving letters were reallocated: Bulgarian kept the big yus in its orthography until the middle of the twentieth century, writing by then a vowel that was no longer nasal at all. And in the East Slavic tradition the little yus was kept for its shape and given the work of a different letter entirely.
The iotified pair fared worst. They were the rarest to begin with, they were the most easily confused with their plain counterparts, and when the nasal vowels went there was nothing left for them to be reassigned to.
The pattern is a general one for alphabets that outlive the sound system they were cut for. Letters are not removed in proportion to how redundant they are. They survive according to how useful their shapes are for other purposes, how much of the numeral system depends on them, and how strongly the institutions using them resist change. Sound change proposes; orthography, printing and the church dispose.