Looking at export_descr_guilds, it becomes clear why it's easier to get Hospitallers than it is getting Templars: the only ways to increase the chance for templars are a) have a general join a crusade (+25) and b) train templars (+10 for the settlement, +1 overall) for which you have to have the guild already. For St. John's, you also get bonuses for declaring war on muslims (+10) and having muslim neigbors (+10 for the settlement, +1 overall). As I found this a bit 'unfair' towards the Templars, I changed the file to give those bonuses to them, too. After all, they were a military order and fought against the muslims.
Added lines are in bold:
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Trigger 0170_Declare_War_Muslim_Faction
WhenToTest FactionWarDeclared
Condition TargetFactionReligion islam
and FactionReligion catholic
Guild st_johns_chapter_house a 10
Guild templars_chapter_house a 10;------------------------------------------
Trigger 0171_Muslim_Neighbours
WhenToTest SettlementTurnStart
Condition NeighbourReligion islam > 50
and FactionReligion catholic
Guild st_johns_chapter_house s 10
Guild st_johns_chapter_house o 1
Guild templars_chapter_house s 10
Guild templars_chapter_house o 1
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