I think the easiest way would be for something different to happen at Nootka in the 1790's. Some sort of deal where the British give up claims to the Pacific Northwest, in exchange for something (I really don't know what - Fernando Po?)
The Spanish and then the Mexican would really have to invest in the coast from that point on (which they had never shown much interest in), and then Mexico could buy Alaska from Russia in the 1860s.
How about the British get Louisiana from Spain?
Far too good of a deal for the British. And it may cause war with the US.