Well, I got a bit bored of myself calling crusades and almost never anyone joining in. Looks like if the pope calls one everyone joins.
So...how do I make the pope call crusades OR make more factions join in my crusades? Faction standing / Global reputation does not seem to play a role in that as far as I can tell.

Only reference I found was this:

Code:
<crusades>
      <required_jihad_piety int="4"/>
      <max_disband_progress float="20.0"/>
      <near_target_no_disband_distance float="4.0"/>
      <disband_progress_window float="3"/>
      <crusade_called_start_turn float="20"/>
      <jihad_called_start_turn float="20"/>
      <movement_points_modifier float="2.0"/>
   </crusades>
in the descr_campaign_db.xml
I'm not sure what the called_start_turn means, but it certainly does not mean the pope calls a crusade at that turn/year count, as I tried it with 10 instead of 20 (i.e. it will then either mean 10 years or 10 turns)