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.