Think mainstream. Think offices with thousands of PCs.
S'all about the value of the land it occupies. Why skyskrapers instead of sprawling estates of houses?
Desk space = money. Bigger the required desk, bigger the required floorspace.
Towers simply take up less square footage in floor / desk space (vertical height is inconsequential).
Mainstream dictates the market - Towers become the mainstream. Tis all just a trend.
However, the same restraints come into consideration for home users - takes up less room in the bedroom... whatever...
End of the day, the non-existant "ideal overclocker" who's after full thermal performance simply doesn't use a case at all. Any case is an airflow restriction. Any airflow restriction decreases performance of anything attached to that case.