A lot of it has to do with branding, and there are some more minor hardware concerns. The branding aspect comes from seeing an entire trading floor with the bra

A lot of it has to do with branding, and there are some more minor hardware concerns. The branding aspect comes from seeing an entire trading floor with the branded monitor/keyboard. It is recognizable anywhere you go. The keyboard has special keys which are used by software features for which the user base has acquired muscle memory (they do help you become a faster user). Also things such as embedded speakers in the keyboard so that terminal A/V can be played anywhere because customers computers might not (it is common) have speakers. Also, wrt to being a web app, there are users and then there are users. Sure, certain customers' usage might be fine for the web (and it can be run remote from anywhere on the web for the obvious reduced performance due to remoting), but receiving 50k+ ticks a second and pumping screen updates to 8 monitors in a trading situation is not something a browser is good at.