They could have written around this very easily:
The bomber itself was thus designed to work solely by being a flying brick that got in REALLY close to the target and then deliver a large number of the exceptionally volatile explosives at point blank range.
The bombers move so freaking slow because of their experimental ridiculously strong shield generators that provide nearly as much protection as a full capital ship but can only be used with minimal ion thrusters and thus very very low acceleration / deceleration. These same shield generators could normally only be overcome by capital laser batteries (which is why Poe takes those out on the dorsal side of the dreadnaught) or heavy sustained fire from ships. This was necessary because the payload had to be delivered at point blank range.
The 'bombs' themselves were, in truth, just mines that sacrificed propulsion for containment of a non-conventional explosive that was too unstable beyond a critical mass to be delivered in a single kinetic missile. The only way to deliver the explosive was through a large number of smaller containment spheres that, when delivered to the target simultaneously, provided an almost unparalleled destructive potential from a relatively small form factor. Using the explosive compound in missiles was dangerous both because of the need to balance containment with propulsion systems and because of the sheer number of sub-critical mass units of the explosive that would be needed to scale the resulting detonation. Attempts to use the explosive on missile systems required very complex 'time on target' delivery and was overly reliant on breaching point defense systems designed specifically to eliminate incoming projectiles.
The critical weakness in the Republic Bombers, and why they were scrapped thus affording the Resistance the ability to procure them, was that in order to deliver their payload they needed to re-attenuate the kinetic-deflector array before the payload could be delivered. As the bombers approach in formation one gets taken out by getting crashed into which then causes a cascade failure as more and more of the bombers get blown to bits. Paige's bomber, because the bombardier was dead, had not yet attenuated the deflector and thus was the only one remaining to deliver the payload.
I'm not saying this wasn't still a silly choice based around the need to have Paige's scene play out as it did. I would much rather have seen a whole squadron of B-Wings tear shit up...but if one really wanted this to play out as it did there are always creative ways to reshape the narrative when working with what is essentially fantasy.