A lot of bikes were still using slack frames and long wheelbases into the 70s. Especially lower priced bikes. Huffy and Columbia were building bikes with 68° frame angles, and on shorter frames most companies used such slack angles to help fit standard sized wheels on as small a frame as possible.

The inconvenience of the high top tube was probably not immediately realized, people having just come from high wheel bikes - also in theory I suppose it makes for a stronger bike overall - less stem and seat post to flex.