Because standard mint sets weren't issued that year and high quality examples are rare. You either had to drive to Philly to get a souvenir set from the mint gift shop or pull a decent 'looker' out of the change. Folks back in the eighties weren't saving large numbers of these coins so decent strikes of the coin command more moola.........
People just didn't collect moderns in 1983. It's a wonder the mint could sell so many mint sets but lots of collectors had found they could buy five of them and sell four at a profit while keeping one as well. When the mint sets were discontinued in 1982 few noticed and fewer cared. People weren't rushing to the banks to set aside rolls and bags so it was largely a non-event. A few companies made privately issued mint sets but these were costly and had just regular coins in them. Relatively little has changed over the years and these coins still get very little demand but the demand is larger than the even tinier supply. Finding uncirculated coins for '82/ '83 is tough and you'll have to pay a few dollars for most of them. If these coins had much demand they'd cost a lot more than a few dollars.
I had an opportunity to buy one of these once. It was a little out of my range at the time, though. Great coin.
