They could be waiting to get a few more API integrations on the...

They could be waiting to get a few more API integrations on the board first to try and cut development costs down.
Apps aren't cheap - nor are half decent developers and given the space they are in - makes it even more expensive given the security they would need to develop in the app.
Having a responsive website that can cater to both web and phones is by far the cheaper option.
Also, depends on how they view an app in their business model - an app like this may not be used everyday, so the development and then enhancement costs each time to update security and api integrations could outweigh the long term benefit...
All IMO and the rest...