Cityjet would need a 2nd aircraft overnighting in Cork to serve Brussels or Frankfurt. I couldn't imagine they would commit to that without having significant data from the London City route to suggest a 2nd aircraft is financially viable.
The reason why La Rochelle and Nantes are easy is because they allow the aircraft to be used during the period London City is closed (13:00 Saturday to 12:30 Sunday). Those timings are ideal for leisure routes but pretty useless for BRU and FRA, which would have a predominantly business/connecting market.
Also, any airline thinking of going to Brussels will most likely wait until the route is gone for 12 months so that the route incentives kick in again (although that is admittedly waivable if the airport authorities regard that it provides a significant commercial benefit).