Let's look at a practical reason.
The Vibranium in Howard Stark's Shield was mined at some undetermined date, and processed by Howard into the shield in the late 1930s early 1940s. It has been frozen for decades, and has received plenty of punishment from physical, electrical/magical, and laser/repulser sources. As far as we know, Howard Stark only had one crack at it. Sure he's a genius, but nothing is done perfectly the first time. He also makes it as a defensive tool.
The Vibranium in T'Challa's claws were mined and processed by Wakandans at some unknown date. Likely made for T'Challa, as his father was not the same size as him. Wakandans have been working with Vibranium for, well, forever, and are typically shown to have technology to outpaces normal modern technology. They are also intended to be used offensively by a warrior.
In the comics and cartoons, Wakandan technology is one if not the most advanced on the Planet. Tony Stark's tech doesn't even come close. Wakandans use vibranium in all their weapons, and have applied technology to it. It is often electrified and has active electronics in it, making it deadlier than normal.
The simple answer is this: The Black Panther's vibranium claws are newer, designed by experienced users for offensive purposes. This is why they can scratch and gouge an older, inferior defensive shield.
At a minimum, the claws are decades more advanced metallurgically than the shield. Worst case, they are decades more advance in metallurgy, active electronics, and built by skilled weapon smiths instead of an amateur technologist.
As Will Smith would say, Cap's shield is the old and busted, T'Challa's claws are the new hotness.