I think the early BA53/52 rotation has a lot of do with connections. Lunchtime is a big peak in SEA with Alaska, and it connects reasonably well to long haul overnights (like DEL/BOM) at Heathrow. Air France, Lufthansa, and Delta’s AMS flights all follow a similar early schedule. It felt like that was the mix onboard too.
They also run similar timed flights to IAH, MIA etc which are similar distances but the smaller time difference makes it easier. BA52 are all wheels up a similar time to BA294 and BA206.
Definitely prefer the BA49/48 but it’s always been much more expensive.