Hard doesn't necessary means good. Harder crystal tend to shatter more easily, despite being more scratch resistant.
Many watch have domed crystal, and how well Corning glass perform when domed is unknown. Deflection and distortion will be very unpleasing.
And there is also the look. The crystal of a watch plays an important role in the final appearance, and should never be underestimated. Omega Speedy Pro & Longines Silver Arrow are two fine example, both still utilising hesalite crystal, just so that they look right.