It kind of had to do with NBC's relationship with Ansin, which was terrible despite Ansin being super loyal to NBC. WSVN at the time was a 3rd placer which pre-empted NBC shows during a time when NBC was less tolerant and NBC thought they could do better and bought WTVJ in 1987, they waited 1 year and a half due to Ansin getting NBC to honor out their contract.
WSVN turned out through failed CBS negotiations which resulted in them buying the then WCIX from TVX a Fox affiliate. Ultimately, WSVN had managed to propel itself in the ratings thanks to its highly successful and unique news operation while WTVJ fell due to finding itself under WSVN's shadow and NBC's inconsistent direction whether to make the news department more traditional or more tabloid.
While NBC affiliated with WHDH at the time, their relationship still felt unstable and after speaking out against the stupidity of the 10PM Jay Leno show, managed to tank their relationship despite the overall failure of the 10PM Jay Leno show. 7 years later, NBC created a station (WBTS) out of thin air, leaving WHDH to become an independent similar to WSVN.
Compared to Nexstar which would involve affiliation switches the likes of which haven't been seen since 1994-1996, Ansin looks tame by comparison.