WCVB 5 was signed on in 1971 by local owners who won the license challenge for WHDH TV Channel 5's owners. Channel 5 was locally owned until 1982. Metromedia (who at the time owned 5 WNEW TV New York (Ind), 11 KTTV LA (Ind), 5 WTTG DC (Ind), 19 WXIX Cincinnati (Ind), 26 KRIV Houston (Ind), 9 KMBC Kansas City (ABC), and 11 WTCN Minneapolis (NBC - formerly an independent till 1979) ). When buying Channel 5, they sold 11 WTCN to Gannett (who renamed the station WUSA and eventually moved those calls to their acquired DC station and renamed 11 Minneapolis KARE) and then sold 9 KMBC Kansas City to Hearst. They also sold 19 WXIX Cincinnati to Malrite. In march of 1983, Metromedia then bought 32 WFLD from Field. They overhauled weekday mornings and prime time replacing some sitcoms and drama shows with first run Metromedia owned syndicated shows not on in Chicago. They also changed graphics, announcers, and overall presentation on that station. In 1983 they bought 33 KNBN and flipped that station in 1984 from Business News by day and Spanish programming by night to general entertainment including cartoons, game shows, first run Metromedia produced syndicated shows, drama shows, and eventually more cartoons and sitcoms. So Metromedia sold 3 stations in 82-83 to buy three other stations.
They narrowly outbid Hearst for Channel 5 Boston in 1982 and at the time sold Hearst KMBC with a promise that if Channel 5 Boston was ever put on the market - they would offer it to hearst first. When Metromedia sold the TV division to Fox in 1986, Channel 5 Boston was excluded and sold to hearst. Fox then bought Channel 25 Boston from Pat Robertson with most (not all) syndicated shows included. Fox overhauled Channel 25's presentation as well with new calls, graphics, and announcers. Fox likely regretted not buying WCVB in 1986 as they had become more news based over the years. But chances are Fox getting WCVB was not an option and Hearst had first dibs on WCVB. that is my guess