So I don't know much about epic 40k or 40k in general from the time period this game is from, but is there an actual reason why all of the ork infantry have bolters instead of shootas and the like?

Simple put, it was 3rd edition 40k/gorkamorka that introduced sluggas etc. before hand in 2nd edition and earlier orks used bolter weapons :(

Final liberation is abit of a mash up of 2nd edition epic (space marine) and (3rd edition) epic 40k..

They used to use Bolters in 1st and 2nd edition of Wh40k. Also, they used to have BS3 back then.

No they had mechanics with genetical memories about how to build stuff that did all their weapons then. Ordinary Dakka weapons wouldn't do jack shit against many enemies back then so the Orks more or less had to use bolters.

Mechanical abstraction, I'd guess - "if it has the firepower of a Bolter, treat it as a Bolter". The actual Orks likely have a wider variety of gear of varying performance, but its easier to work out combat if its assumed to perform as the average case.

They kept human slaves for making STC weapons for them or demanded arms as a form of tribute from human worlds.
Mekaniaks were creating kustom and primitive weapons and kustomizing equipment received/looted from aliens.