IBM has done this, despite it being an inferior technical solution, because it binds those who choose it to IBM hardware.
Programming currency operations with decimal floating-point numbers is easier for naive programmers.
Even on IBM mainframes, implementing currency operations using 64-bit or 128-bit integers is much faster and less resource-consuming than with decimal numbers, but the implementation of some of the operations can be a little tricky, when it must be guaranteed that no loss of precision may occur.
I think that I might have seen recently an announcement from someone else than IBM who has introduced hardware support for decimal floating-point numbers, perhaps from Fujitsu. In any case, whoever introduces such hardware support does it to lure some customers to migrate from IBM to them, and not because it were a good solution for implementing operations with money.