It's partly historical: these applications already exist on mainframes, and it would be very expensive to change, so they don't. But mainframes have a lot of a

It's partly historical: these applications already exist on mainframes, and it would be very expensive to change, so they don't. But mainframes have a lot of advantages because they are integrated systems designed for exactly the sort of high-throughput tasks that businesses use them for. Their (single-threaded) processing power is good but not outstanding; where they really shine is parallel I/O performance which lets you execute large numbers of operations per second, or distribute workloads across large numbers of processors with extremely high bandwidth inter-processor communication. They're also designed to be highly reliable, redundant and modular. You can do things like upgrade and replace processors, RAM, and disk drives without interrupting applications. And speaking of processors and RAM, modern mainframes can scale to >100 processors and terabytes of RAM in one system. Yes, you can build systems with similar specs and capabilities with clusters of commodity hardware, but then you have the complexity of managing a distributed system. With a mainframe, you have a single beefy machine. You'll also have a support contract with the manufacturer to call when things break, so they can be responsible for doing any complicated maintenance. On the downside, of course, they're expensive in the "Q: how much does it cost? A: how much do you have?" way that enterprise pricing works.