My theory is they closed registration because the new influx of traffic (although i doubt much demand to join now) would require the owners to monitor the bandwidth and would have to spend money to upgrade the servers.
Right now they’re maximizing profits. Less active posters than ever before but still a good daily visitor core, so probably have a fairly inexpensive server while still pulling in ad revenue from visitors. This site has lost many features and has gained no new features in return. My guess is the now lost features (gambling, stock market, custom smilies per poster, custom groups, etc) used the most bandwidth and storage so removing those features and downgrading but keeping most the active posters = most profit. Probably costs $100 a month to operate this site at this point, while probably still generates $2-10k a month in ad revenue.
they put this site on cruise control until it eventually fizzles out in the next few years.