Possible reasons:

1. The traditional French component suppliers were in a bad state at the time.
2. The French groups were a mix and match from different suppliers with no uniform cosmetics.
3. Ofmega offered a reasonably price alternative and offered a range of cohesive groups.
4. Ofmega wasn't German based, like Sachs, which by this time owned Huret.
5. Ofmega, being European would appeal more to competition segment of the market than SunTour or Shimano
6. The plastic Ofmega Mistral derailleurs, would appeal to the French market, who favoured light weight and offered some continuity for Simplex users.

By getting Ofmega on board with at least one bicycle, Gitane, they were establishing a relationship in case there other sources disappeared or became less popular. Double sourcing is a common ploy. You give the 2nd source just enough business to keep them on board, in case you primary supplier can't supply you, loses market popularity etc. The American brands used to do this with their Japanese sources. When SunTour was popular they'd offer at least one model with Shimano. When the market shifted to Shimano, they offered at least one SunTour model, in case the market shifted again. It's the cycling equivalent of not putting all your eggs in one basket.