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.

It's a mistake to think about Bloomberg as an application, it's much closer to a platform which hosts a wide variety of applications (that just all happened to be made by Bloomberg). Some of the apps might work fine as web apps, but if you have for example an app that requires displaying a large number of updates (for example realtime pricing of the S&P 500) then html/javascript just doesn't have the performance.

Also most of the big clients have physical connections to Bloomberg directly rather than going over the internet (for redundancy, security and performance reasons), and you need to optimize the data going over those connections. If you have a few hundred people at one client all watching 1000 prices in real-time you don't want to send all that data to every user separately so you need to use a fan-out node located at the client site.

These are just a few of the kind of complications you'd need to deal with if you wanted to build a web app that did the same thing. I doubt there's a single web app in existence which has anywhere near the level complexity that Bloomberg on the web would have.

It might be possible eventually but we're still someway off.

They have a web-launched SaaS version, too, and they're trying to get everybody to switch to it because the pricing model is per-user instead of per-machine.

This is definitely true, the tech Bloomberg uses was novel in the 80s, but the functionality of a modern browser now outstrips that of the terminal program. There's actually some functions in the Bloomberg Terminal these days which just fire up an embedded browser to render the content!