Most are paid in LBP, it's the difficulty converting that to USD (to then spend it on travel, among other things) that's the main difficulty.
That said, I don't think this is the main reason for the suspension, rather just one step too far. Back when BA started the route, Y fares were often double what they are today. That isn't due to a lack of demand but increased competition on many BEY routes, as well as MEA's double daily services to LHR. It's been interesting to see BA's price aligning to MEA's over the years, introduction of one-way fares (even ex-LHR), HBO fares, etc. Furthermore, MEA has increased interline agreements with connections out of their European gateways - you can easily build up a BEY-ME-LHR-AA-JFK ticket, for example. With VS/AF cozying up, BEY-ME-LHR-AF(VS)-BOS is also a possibility, for example. Premium US connections are/were BA's main goal for the BEY route.
BA was just starting to prove less popular - especially in the last year or two - with erratic schedules, somewhat clapped out cabins, long connections at LHR on the way to BEY for many US-originating pax, perpetually A10 at LHR, and arguably an inferior product to MEA, at least in Y. Add in increased competition and improved products on competitors, mainly CDG, poor aircraft utilisation, double overnight layover, the occasional (IMO overreaction) overnight hop over to LCA and back when things heat up somewhat, and there you have it.