Why might Eliot have called this a love song?

Because he wrote it for someone he loves.

If you mean Ts Eliot then "The Love Song of J. Alfred Prufrock" is a modernist poem. T. S. Eliot, was an American who moved to Britain in 1914. Eliot wrote most of "Prufrock" when he was 22 years old, in the years before the start of World War I.