When they add adjuvant to a vaccine it allows them to use less antigen per dose. These adjuvants have also been linked to many bad side effects.
The amount of vaccine that can be made is limited by the amount of antigen (virus) that is available. The virus (antigen) is grown in eggs and it takes time to grow and purify it for use in the vaccine. A company can make more vaccine, using less antigen, when adjuvant is added.
To put it another way, it is cheaper and faster for them to mass produce a vaccine with adjuvant than without. The peons get the cheap stuff.