Beginning with the advent of the bicycle, the headtube length was more or less an afterthought. As bicycle frame geometry was originally devised, the head tube was exactly long enough to allow for the appropriate seattube length to allow a level top tube to bridge the gap nicely
I mention 29" mountain bikes because of a happy accident that occured when tires all of the sudden magically grew by 3 inches. This created (automatically) a front end height of....you guessed it!: old front end height plus 3 inches.
Why is this additional front end height important? Because when your body has exactly 5 weight receptors: 2 hands, 2 feet, and one backside, it is imperative that you DO NOT overly stress one or two of those pressure points. All this aside, proper weighting of a rider's hands also provides for effective steering and overall control of the bike. Austinbikes currently sells several models of road bicycles and 29" mountain bikes to allow for this optimal pressure balance between ALL of a rider's contact points. All the bicycles listed below have well apportioned headtubes to allow for a multitude of fits from uber-racer to Hill Country tourist.
the Scott CR1
the Fuji Grand Fondo
the Ridley line of bicycles