According to Compustat Technical Guide:
"On June 30, 1997, the Financial Accounting Standards Board (FASB) issued the pronouncement for the Statement of Financial Accounting Standards No. 131, Disclosure about Segments of an Enterprise and Related Information (SFAS 131). The statement establishes the standards for the way companies will report information related to operating segments in their annual and interim financial reports and supersedes SFAS 14, the basis for the COMPUSTAT Business Information Files. Due to the changes required under SFAS 131 for the reporting of business segments, a revised Business Information File has been created to allow for the collection of data reported under the new standard.
SFAS 131 has elected to make the new reporting more relevant than consistent or reliable. It specifically states that both relevance and comparability will not be achievable in all cases, and audit systems should be the overriding concern. Because FASB has determined that internal performance and audit systems will be the source for the new segment data, this data will not need to reconcile to the consolidated statements."
Because of the change due to SFAS 131, Compustat had to create new Segments data. Obviously there wouldn't be data prior to 1997 (the date of SFAS 131), so Compustat went back and did research to fill in pre 1997 segment data that would conform to SFAS 131. But due to the large amount of work required to "backfill" the pre 1997 data, Compustat only went back as far as 1990. That's why you don't see data before 1990 for SIC and NAICS at the segment level.