They want people using their app, for some reason I don't understand. Maybe to make ad-blocking harder by getting people off computers where it's easy (most mobile browsers don't support plugins, so it can't be about them).
Ironically, every time I click their “open in app” link it’s drips me straight to the App Store despite me having the app installed.
The old design was optimized for usability, the new is optimized for ad revenue and/or for pushing people on the app.