Why does Shutterstock keep Big Stock around? They�ve owned it for years; I would have expected them to merge the sites ages ago, but they haven�t.

It's very common for companies to aquire other brands and then not merge them. I could think of plenty of exampled in Europe  (L'Oreal/The Body Shop, MediaMarkt/Saturn, S.Oliver/Copenhagen Studios, etc...)

The point is too keep the customer base of the other company.  The customers that are with Bigstock and not Shutterstock will have their reasons - I don't know them, but if you just wanted the most selection of images for the cheapest prices you would go to SS or iStock. So it's something else the customers like there, it may be a different search function/algorthm they prefer or the credit-based system. Whatever is keeping the (not so many) customers on Bigstock, it's something SS doesn't have, or they would probably already have switched over.
If SS merged with Bigstock they would not gain new customers (as Bigstock customers are already their customers), but risk losing some to other microstock agencies. They'd also not really gain new content as pretty much everything that's on Bigstock most likely is already on SS as well. There is more to gain by keeping the sites seperate.