Partly because the first segment was with BA but mainly because I am spending more than a year travelling so my Itinerary is going to be changed a lot as half the ticket will not be confirmed until Oct this year when I can plan the US portion of the ticket (I start in Oslo in Jan 2026 and end in Oslo Jan 2027). Dealing with a known entity, roughly in my current timezone (even if they are awful), was part of my reasoning.

Second, I was going to book with Trailfinders and they seemed good when I rang them first time, but then they told me that although date changes were "free" Trailfinders would charge me 250 GBP (maybe it was 250 USD but whatever) for every change I made - which was their admin fee. Given I will make 5 or 6 changes to this ticket that was never going to work.