But I can see feeder flights from various locations in Scandinavia and Eastern Europe.
Why?
To fly for an hour or more so that you can to go through UK immigration on arrival in EDI & then effectively go through the US immigration process immediately thereafter just so you can then cool your heels for the ever-increasing minimum transit inter flight time all in the effort to stroll off the plane once you've spent another eight hours plus crossing the pond really doesn't seem like a selling point to me when on the great circle you're not much more that the same eight or so hours from most of Europe to the US.
Even if the direct flight from the generic European destination is 3 hours longer than the EDI-US flight time it'd still be quicker to fly direct & clear on arrival than stop for pre-clearance here. I've been looking into what I can arrange for my own transatlantic travel this month & there's no direct flights this time of year & the shortest layover that I can book at an intermediary airport is 105 minutes. So, 2 hour flight from Europe to EDI, an hour and three quarters in the inevitable shopping mall and another eight hours across the pond versus even a 10 hour direct flight you're still looking at a quicker direct flight, even if the US immigration etc takes over an hour.
for many years I used to have to virtually commute to IAH for part of each year & I have to say that pre-clearance from my own experience was almost wholly irrelevant. Overall,my abiding preference was the original Delta 767 flight to ATL, clear there and hop on the next flight to IAH. The time between flights was really nicely judged, and the TSA there was really switched on, to allow you to clear without any real delay or risk of missing your connection and also it was good get up and move about freely. Second preference became the UA 757 to EWR, clear there and on to IAH. Slower and a bit of a faff changing terminals at EWR.
JFK was an unpleasant necessity on occasion & the BA/Heathrow clown show to anywhere else should frankly be buried at midnight with a stake through their hearts and their heads removed. [I am not a fan of London Airport] Clearance for international arrival at IAH I have to admit was hit & miss, depending on exactly what terminal you arrived at / were re-directed to once on foot & just how many jumbos, treble-7's etc. had all showed up at the same time.
As some of the others have pointed out, pre-clearance doesn't have any influence on the airline's route development plans, and from a passenger perspective it's a necessary evil that
has to happen
somewhere.
Whilst not wishing to stir the pot further, should "The Donald" by some global misfortune manage to get his feet under the oval office desk again, expect a reduction in pre-clearance facilities, not an expansion. [Nor am I a fan of the delusional snake oil salesman.]