The early booking figures were very poor last year for SFB and POP (feb-apr), meaning First Choice ended up selling most seats at lower late deal prices, this in turn ment that they weren't getting the money needed to sustain the routes during feb-apr, it costs them slightly more to operate these routes as they need extra fuel to go via MAN/GLA.
The a/c could and has last summer on one occasion made it direct to SFB, but they can't put any cargo on it as it would be too heavy to go direct off BRS runway, and cargo is where the money usually is for these longhaul flights, hence why they operate the SFB via MAN and the CUN via GLA, Although I dont think CUN would make it direct even with no cargo.