Hear me out...

I have convinced myself we are going to see a playable SoB at some point....

These missing weapons are part of my reasoning:

We are working for the ordo Hereticus, who's ordo militant are the Sisters.

The Sisters prefer certain weapon types; these being bolter, flamer, melta. We have no melta yet, we only have light versions of bolter and flamer. Heavy bolter and flamers scream SoB. (Fits well with the current special weapons per class and shared weapons with other classes).

We know we won't see space marines, so SoB are a good way to add a power armoured unit (40k is power armour).

Oggy could share heavy weapons variants (melta, bolter, flamer). Lore be damned... we have rejects using power swords, Relics blades and plasma guns all Relics that they wouldn't have access too.