[usrusr]

> It needs a class of wealthy people who can buy armour and horses.

A chicken/egg cycle, initialized by temporary imbalances that created a stable system: an increased advantage of cavalry over infantry drives up the price for "protection", which in turn feeds the horses. With less advantage of cavalry over infantry, the class system would have been much less pronounced.

In the spirit of "strange women lying in ponds distributing swords": a fluent socio-economic power structure was converted into an intransparent class system self-stabilized by the barrier of entry imposed by the inherent cost of heavy cavalry.