They have those two loadouts so they can have a point to begin with.
While other ships have some laser cannons, they don't have anything else which can be a dedicated anti-fighter ship. The CR90's oadout according to Incredible Cross-Sections was various turbolasers, but that's not consistently maintained in other source material, and that would be worse than useless in the mod. A ship that small with that little anti-ship weaponry wouldn't be worth building at all, especially since it would then be awful against fighters, too. Several different configurations are mentioned in various places, and one of those configurations includes six laser cannon turrets. We went with that one, since it's the only one that actually makes the ship useful in EaW.
The IPVs in the mod are modified in the same way, for the same reason. The Tartan specifically isn't used because it's not really mentioned anywhere other than EaW, and its inclusion in EaW was basically because they modeled a Carrack poorly and decided to rename it rather than remodel it; the code even names it as such. The PA was a late addition to 2.0 made with the resources we had available at the time; once we're able to, we're putting in various other small craft to serve as anti-fighter ships for the factions that currently use the (modified) IPV, which was one of the most common ships available for the Empire, and therefore post-Imperial groups (and modified by us for the same reason we used that specific CR90 loadout).