Because going by ICS/Wongcalcs those little explosions are nothing.
Because the whole symbolic angle is silly, how many times has a modern militaty gone symbolic by using flint-tipped spears and slings?
Ok, 1 ISD, or 2 ISDs, or something would have to drag all those fighters to Mandalore, and the probe droids, ant the cmombat droids, sending all those into Mandalore's atmosphere and down to the planet's surface would probably take just as much time as the ISD orbiting around the planet and hitting most major population centers with turbolasers.
IIRC the Empire never had dedicated carriers of the size that could take in that many tie bombers, so ISDs would have to be it.
I stick to Occam's razor, and the simplest solution here is that the empire's planet-destroying weaponry aside from the Deathstars is pretty underwhelming, thus Wongcalcs/ICS go down the drain IMHO.
And the effects of the First Order Dreadnaught's bombardment canons corroborate that, they were quite puny by comparison to ICS stuff.
And recall that that ship was far larger than an ISD and purposely built to raise planets.