I believe it's because those are coded into the modify proto unit trigger effect. There are still a heck of a lot more feilds that can manipulated through modding, but not in modify proto unit. In fact, just last night, I started working on a proto functions list.
It lists all the available functions that can be used in protox, the general unit info, (maxvelocity, unitaitype, displayednameid, etc.) unittypes, (AbstractInfantry, Dropsite, LogicalType..., etc.) flags, (DoNotDieAtZeroHitPoints, GatherDirectToPlayer, NotPlayerPlaceable, etc.) parameters (Damage, DamageBonus, VolleyMode, Inactive, Persistant, etc. along with showing what those parametres accept, perhaps a "type" and a "value") actions (AreaAttack, Gather, Wander, MoveByGroup, Build, etc.)and "Unused" (DamageMax, and the other parametres/flags that never ended up being put into the game). With a breif description of each tag. Along with unit ID's. So when it's done it will have nearly everything, if not outright everything, that one could possibly ask for.
Grr, one more big thing being added on my "to-do" list .