To prevent the original strains from coming back when immunity to those (eventually) wanes.

We don't want to start going backwards. Plus, there's cross-reactive immunity so that similar mutations can be recognised by the immune system without ever seeing that particular one before.

Keeping a wide breadth of spike mutations allows that to work more effectively.

Edit: u/nomnomnomnomRABIES, the reason is that Flu is an entirely different beast to COVID. Despite all the mutations COVID has gone through, it is not all that different to the original strain (which is a good reason why our immunity still holds so well). Coronaviruses do not mutate all that much, as they have the largest genome of all RNA viruses. COVID is just mutating a bunch, relatively, because of how widespread it is.

Flu, on the other hand, drifts massively, and constantly. There's no point including older strains because it doesn't help you fend off next year's Flu. Maybe once or twice in your life you'll come across a strain that is similar to one you were previously vaccinated against, which is nice, but no point wasting time cramming a Flu vaccine with all these historical Flu strains.