Mäkinen wants to use BOS because they have used them on their Gr.N Subaru, the main reason I think is Lasse Lampi is BOS importer for the nordic countries and Lasse has a long history with Mäkinen.
This is actually something alot of people wonder about as there in all fairness are more established suppliers out ther.. like the ones you mentioned for instance
Citroen buy various parts from BOS for their WRC cars and assembles them themselves(same as they used to do with Exe-Tc in the past until ~2010)