These cities have less frequency compared to say LHR and the flights are relatively short. As they don't have enough aircraft fitted with the new F/J, there is less of a chance of substitution if they keep it on the BRU/GVA routes.
If they used it on a route such as LHR with many different frequencies, it is more likely for frequent substitutions. As LHR might be more "premium", passengers are more likely to be angrier if the aircraft gets substituted.
Once they have enough aircraft with the new product, it is is likely they will get swapped out of BRU and GVA to other routes.