The original place I seen it was on Wikipedia, so far I haven't done any further reading up on it as of yet, but here is the quote from Wiki.
Although the cast members were contracted for eight seasons,
Paramount ended The Next Generation
after seven, disappointing and puzzling some of the actors, and an unusual decision for a successful television show. Although doing so let the studio begin making films using the cast, which it believed would be less successful if the show were still on television,
the main reason was that additional seasons would likely have reduced the show's profitability due to higher cast salaries and a lower price per episode when sold for stripping. The decision also encouraged viewers to watch Star Trek: Deep Space Nine
and the forthcoming Star Trek: Voyager
, both of which were much cheaper to make than The Next Generation
. The show's strong ratings continued to the end; the series finale was ranked No. 2 among all shows that week, between fellow hits Home Improvement