Mid-2010 MacBook Pro - HP ZR30W - mini DisplayPort to DisplayPort?
It's likely that you just have a defective unit. There's no active circuitry in a mini-Displayport to Displayport connector, and a Google search suggests that this monitor has worked with Macs over Displayport (as you would expect), but some people have had resolution detection issues.

If the dual-link adapter is working for you, great; however it may be worth investigating a warranty exchange, as that adapter has not always been reliable for others.