We do actually have multiple flu strains I'm the vaccine. Most of the vaccines now are Quadrivalent, meaning they have four strains in the one vaccine. The problem is that the flu mutates quite rapidly and there are many subtypes of each of the larger virus clades, and predicting which ones will be most active each season is hard. Scaling is a bit hard because it's essentially making 4 vaccines and packaging them in one syringe, so adding more varieties has linear costs but diminishing returns.