Because PoP´s work in progress and the nice feature of enabling KINGS to hire MERCENARY COMPANIES was magically develloped by Mad Vader, highly appreciated, quickly introduced and tested and thus taken off the player´s payroll.
The PC´s usually strong enough as it is and the AI needs all the poor help she can get. Royals are royals. The PC is common as muck, wether he´s a Rogue King or Mercenary Captain. He´s too insignificant to attract the real established Mercenary Companies. And he´s got more than enough new mercenaries to chose from - all the minor factions are out there to be hired in the various heartbeat quests so in short:
The Mercenary hirement system was changed becaused of good old gamebalance and the nice and happy thought of disabling the player to acquire everything shiny, gleamy or glittering on the map.