Looks like the frame was designed for proper head and seat tube length while accommodating less confident riders who jump down from the saddle to straddle the bike when stopping. Less likely to crush Johnson and the twins, compared with a horizontal top tube.

A simple compact frame with sloping top tube would require an unusally long seat post extension for some long-legged riders (my Globe Camel seat post is fully extended for that reason). The Autobike and Cannondale frames appear to be an effort to resolve that issue.