Armies are loyal to either a country, possibly regardless of who sits on the throne, to ideals, possibly regardless of where they lead, or to strong leaders, possibly regardless of their allies. But mercenaries are loyal to the one who hired them. If you hold a contract, you can probably trust that the terms of it will be upheld no matter what. And unlike most regular soldiers, mercenaries have usually seen an actual battle and know how to fight in practice and not just theory. Some might object to loyalty to money over higher ideals, but few deny the effectiveness of mercenary troops. And for those who hold those lofty ideals, many noble families make some of their money by investing in mercenaries.