They are breeding, but the losses outweigh the gains...Long term prognosis for the husbandry of this species: rather doubtful, with Askania Nova as the sole exception.
Principle causes? Saigas are nervous, easily started animals (like pronghorns) with a high tendancy to panic for any reason (may it be a wild fox, waterfowl noisily landing closeby, construction works, fireworks, a stuttering engine...take a pick) and then run blindly into obstacles (such as fences...), breaking leg & neck. Endoparasites have also been a problem in captivity, and the intraspecific (territorial) aggression of males and a pretty high infant mortality aren't helping the cause, either.