1) it's serviced directly from London City airport, which is tiny, but also closest you can get to the city centre by air
2) because of how short that airport is, BA ordered special shortened Airbus A318 that is only made in a full business class configuration(only 32 seats) and it only ever flies on that one route. And even then it's actually too far for its range, so it makes a quick stop in Ireland to refuel before crossing the Atlantic first.
I'm reasonably certain that paying extra for this special flight saves you more than 90 minutes compared to taking a conventional one from Heathrow, no matter which class you'd travel.
It's only too far for its range when flying from an airfield _with a runway the length of City_. From Heathrow it could do the flight non-stop: it just can't take off with so heavy from City.
Of course Heathrow is also far busier than Shannon and far more likely to have delays.
I never understood the skip-security and skip exit-control line: those are public services.
They don't offer the service for free.
The only issue with queue-jumping public disservice is that it discourages fixing the problem in the first place.
* Some low-cost airlines (like EasyJet) are pretty much doing just that, their premium membership give you access to priority lanes without any other significant benefit (since they don't really offer any).
* I don't think anyone wants ticketing to get even more complicated. It's obviously a lot easier and more rewarding for airlines to treat their frequent/business flyers well and make everyone else take the slow lines.
* First and business classes don't fill up the planes, airlines are directly impacted by shitty airports that can't embark/disembark/transfer 80% of their passengers (and luggage) quickly enough. You can't say there's zero incentive to fix the problem.