Older coworker is hoarding information

3 years in as a market risk analyst. Smallish trade shop.

So my company wants to keep me, and keep my happy.. And are trying to accelerate my career.

The market risk manager is close to retirement age. Let's say this person is not well liked, and barely passable at their job. Very rudimentary methods. I firmly believe the company would be way better off if I was in their role. With that being said, I can recognize the manager has some soft skills, and experience is meaningful. 

They are a long tenured employee, and unlikely to be fired. But they are definitely on the fringes of workplace politics. 

The situation I'm in now is that my company wants me to start taking on some of their responsibilities. 

The manager is basically refusing to do this, and is extremely protective of their duties. Like, sensitive to other people even being CCd on dumb emails, and password protecting spreadsheets that aren't confidential...

The crux of it is that nobody would really see this, or quite understand it. They aren't involved enough.

So how can I lobby to keep moving my career on? 

I am not quite sure the right way to lobby for it. 

The answer might just be to look for greener pastures, but my company otherwise is pretty great, and it's difficult to look elsewhere given how niche my product is.