Why did I spin a main bearing?

The crank measured out good so i polished the journals with 1000 grit and a shoestring if i remember right. The engine never went to the machine shop. internally balanced. I did not check the balance. I just did a quick, cheap rebuild because the oil rings were collapsed and it smoked bad. after 40 years that's understandable and it sat for about 20 years before i started it back up. also wanted a little more cam so that was the last straw. I made sure everything went back where it came from though. was running about 3000 rpm and just went to grab 3rd. thanks for the quick responses