There are a couple of possibilities. One, which many may disagree with is, when going with a new cam, it is not a bad idea to replace the distributor gear. The used gear may have had a wear pattern set, that did not work well with the new cam and in turn they ate each other up.
Depending on the new cam, was the distributor gear the correct material? Some cams require a bronze or similar material instead of the factory iron gear.
Last, from personal experience, a "high performance" oil pump can cause the gears to be eaten. Stock configuration pumps work well even for radical engines.
