Many corporate financial software, enterprise resource systems and production control systems have web interfaces. Upgrading the browser may trigger possible incompatibilities with the software in the heart of their business process. In my own environment I have seen large companies going bankrupt during such upgrades, sometimes for reasons like not being able to create invoices during a prolonged period of time.
No large company or institution will upgrade their central software and the browsers to access it if there is no compelling reason to do so. The last compelling reason was the "Millennium Bug". Those companies are now stuck software-wise in the year 2000 and they will stay there for some time as long as economy is not getting better to give them the money and buffer to survive a central software upgrade or incompatibilities with a rolled out browser and that central software.
Here on WebmasterWorld we mostly think of browsers as the access medium to websites. But for many companies it is the access medium to their business process and web surfing is just secondary activity.