IIRC, MiG's were intended for higher altitudes than other Russian A/C. They were discontinued for several reasons.
* They were prone to burning (a bit like the Japanese planes)
* They lacked self sealing tanks (weight saving to help performance at alt).
* Most of the fighting on the east front wasn't at high altitude.
* Other Russian types were superior at low altitude where most of the fighting was going on
* Their production numbers were lower than other types, so in rationilising production, it made more sense to discontinue the MiG's and convert the facilities to the other types than the other way 'round.
There may have been others, and I'm not sure on all of those (particularly the self sealing tanks issue).