As a large bank, BNY Mellon has a significant number of transactional messages that need to be sent every day. They need to be sure that every message will be delivered, and they want to be able to use a single system across all of their platforms.
BNY Mellon uses IBM MQ because it's good at what it does. They know that their transactional data won't get lost and that IBM MQ will work across whatever platforms they use. BNY Mellon also takes advantage of the IBM experts available and uses IBM as a sounding board.