[mardifoufs]

I think that's for Mig-29s specifically. To put it lightly, mig-21s are much older and wouldn't exactly be helpful to Ukraine.

[ersoft]

Also, the romanian Mig-21 are converted to NATO specs, the ukrainean are not able to use them without training. Maybe it is the same issue with the Polish/Bulgarian Mig-29.

[jacquesm]

And some conversion lest Russia thinks that NATO planes are in the theater.