Why did Hubble have to rely on Cepheids to find the distance to M31?

Hubble had managed to locate Cepheids in the arms of M31 (Andromeda) and M33 (Triangulum). He was unable to resolve these galaxy's central regions, or indeed any stars from population II. Hubble’s observational restriction to population I Cepheids would have important ramifications when Shapley’s error was later discovered. Nevertheless, when, "A spiral nebula as a stellar system, Messier 33" was finally published, Hubble estimated it to be at a distance of 263 kpc (Hubble 1926). He also showed that M31 (Andromeda) lay at a similar huge distance, and was more than twice the diameter of M33.