This web server is a rolling archive. Older data are deleted on regular intervals.
For full time series, go to the FTP server:
1) FTP via web-browser based HTTPS service: https://www.star.nesdis.noaa.gov/pub/socd1/ecn/data
2) FTP via your own FTP software: ftp.star.nesdis.noaa.gov/pub/socd1/ecn/data