Picard ruled Toral's challenge valid, an assassin would kill Gowron before the fight.
Why wouldn't the assasin kill him anyway, besides Gowron has to have best security in the quadrant.
Gowron actually had some shoddy security in those two episodes. In Redemption 2 a random guy just walks up, draws a knife and fights Gowron for example. Even a relative nobody like Kurn boasted about being able to kill Gowron and I don't think it was idle.
As to why not have Gowron killed before Toral's challenge? I would assume it is because Picard didn't rule Toral's challenge for the leadership valid. As the arbiter he decided who could and couldn't challenge. If they killed Gowron before Toral's challenge was approved, they'd probably have to go through all that cerimony they did in Reunion again, which would probably just result in a civil war anyways after a long period of waiting for new challengers and the such.
If Picard had ruled the challenge valid, then the assassin would have killed Gowron and Toral would be left as the only challenger for leadership, much like it was with Gowron when Worf killed Duras.
As it happens, Picard found Toral's challenge invalid and the Duras Sisters pretty much stopped bothering with Toral as a proxy and openly tried to overthrow Gowron with Romulan help.