There's no real "why" here; vowels tend to change a lot over time, and sometimes diphthongs turn into monophthongs, or vice versa. It's just an accident of fate.
/oi/ in particular,
/i:/ is a pretty natural thing for it to turn into; you can think of it as the second element "taking over" the whole diphthong, or the first element assimilating completely into the second. You see this in Old Latin and in Proto-Slavic, as you mention, but there are other possibilities too: Greek turned it into
/ø(:)/ (taking the rounded-ness and mid-ness from the first element and the front-ness from the second), then
/y/ and eventually
/i/, and post-Classical Latin turned it into
/oe/ and then to
/e:/. Or it can just survive for a long time without changing into anything else; English has preserved
/oi/ remarkably well across time and dialects, compared to what's happened with other vowels and diphthongs.