ALL of the other passes were attack runs; the Klingons fired at them every time, chipping away at their defenses. Kirk was able to overcome them by playing possum and then smacking them with torpedoes when they dropped their guard.
But the thing is, the Klingons were only able to "chip away" because of the sabotage and the subsequent playing possum ruse. We got no evidence that this would have been possible against a non-sabotaged ship.
Basically, there are just three times a single battle cruiser opts to challenge an unhurt Enterprise
. The first is in "More Tribbles", and hinges on the use of another dirty trick, the stasis field. The second is in "Time Trap", but only because the lead ship springs a trap later involving two others. The third is in ST6:TUC itself, but in a situation where there are no other options left (and where the challenge is actually part of the greater plot - the Klingons hope to lose their ship to Kirk!).
That a small BoP challenges Kirk on (at least) two occasions (as the "Errand of Mercy" teaser is a likely third case) nevertheless is probably more an indication of such a ship being more expendable than a battle cruiser than of such a ship having better odds.