The 1989 Uncirculated Silver Eagle is common in the series. However, it does have a large collector base as Silver Eagles have become extremely popular over the years. So, some of the earlier issues in the series have a larger collector demand, especially since fewer were produced compared to more recent years.
Finding nice well struck examples, with clean surfaces for this issue can be difficult. So many collectors will settle for an raw or MS68 or even low quality MS69 examples. Smarter collectors will usually try to find very nice problem free examples and many times are able to purchase them at no extra premium than an average MS69 example. With enough searching, a nice MS69 example can be found. On the other hand, perfect MS70 graded examples are very difficult to find and command a very significant premium over MS69 prices.