The Codex merely gives you the default options, not pandering to special cases, and hence stressing certain differences in style between the Ordos. So you may have few Inquisitors in the other Ordos having Terminator armour, just like you may have Inquisitors in the Malleus not having it.

Most Inquisitors seem to prefer the stealthy approach, and as redkommando mentioned, Terminator armour doesn't lend itself well to that.

It's more likely a personal thing, a mixture of connections to secure the necessary deals to have one such suit constructed, as well as the simple willingness to don and use one. I would also say that, theoretically, Ordo affiliation has little to do with access to Terminator armour. I'm building an overall 'take all comers' type army, though usual opponents for the army at this point will be infantry heavy and armour-light, though a lot of Power Armour. Unlike the Marine Terminators, his (or hers) is a custom job uniquely crafted by the Mechanicus' artificers, of which likely there is only a single examplar in the entire galaxy. The Saturnine Terminator Armour that would one day become synonamous with the cold and unyielding IVth Legion was first envisioned in the hellish wars of reunification fought over the carcass of old earth, the Mk1 Thunder Armour being deemed insufficient for what lay ahead, though these projects never came to fruition, abandoned shortly after the Emperor and his chosen warriors left the cradle. I'm about to build a 5-man (Tactical) Terminator Squad for 8th - but I'm not sure which options to take - how many chainfists, or Flamer vs Assault Cannon. The most obvious thing would likely be the Inquisitor's suit being decked out in Inquisition iconography and richly ornamented.

Granted, Inquisitors are not as 'wide' as Space Marines, but Terminator armour is so thick and pimped out that there would probably be little difference. I'd say that Terminator armour would look almost as bulky as the Astartes variant - after all, it's basically the same thing, just a little scaled down (assuming your Inquisitor is smaller than ~2.1m).

Warhammer 40k terminator armor