probably because there’s not enough demand from the west only. If you’re starting SFO-EU somewhere, you need enough traffic from the pacific states and maybe a bit of traffic close enough to the east that it makes a sensible connection. Likewise, if you have a destination in Asia that only has one flight, you’re going to send it from west coast, because someone in Chicago isn’t going to backtrack to say, EWR to catch a flight an asian city, much less someone in SFO.

Part of it is also going to be demographics, and where the pax are coming from. To most EU ports, and not saying that there isn’t some exceptions, there is going to be more of that traffic based on east coast vs. west. If theres enough demand for flights from other hubs also, you can start adding capacity to hubs farther away. Not that there are aren’t people coming from them at all, but you can also connect SFO folks to PRG via EWR, but no one in their right mind would do EWR-SFO-PRG - unless you count the minuscule amount of folks on FT looking at MRs. it also costs a heck of a lot more to route folks on a routing that flying 8000 miles out of the way (says a guy who has twice routed CVG-IAH -YYZ on last minute awards since that was all that was available, ensuring I flew 800+ miles south to get on a flight to YYZ that basically overflies CVG anyway).