Windows NT is supposed to be a mature product. Moreover, Visa employs some of the best network security staff available on the market. People continually come up with absurd justifications for why their company should stick with an OS they know is flawed. But these arguments hold little weight when no matter how long the security staff trains, they just cannot keep the hackers out. The real reason is more subtle.
Humans are just instinctively terrified of change.
Timidity is the glue that keeps society and civilization the way it is. People are so paralyzed by a fear of change that change rarely happens. Customers cannot help being suckered by Microsoft, programmers can't help making mistakes, and I can't help embarrassing myself on trains. But of course, never let it be said that Microsoft does not learn from its mistakes. Recently the company announced as one of the great features of the new MS Office XP suite that its Clippy the paper clip help system will not be appearing on our screens anymore.