FRA needs interlining traffic.
As when BACon operated the routes, FRA as a point destination doesn't generate enough traffic. I'm told BM are working on codeshares with LH, which hopefully would bring the volume up.
The prevailing assumption that interlined/codeshared traffic isn't good yield isn't necessarily true. I just booked BRS-FRA-BKK-BNE, and from the way the fare is constructed, it looks like BM is getting the BRS-FRA fare as if I'd bought it from them directly. (aka, on my travel dates, about £250).
I hope Helvetic will sort out codeshares with LX for the ZRH route.