The temperature also matters. If you need at least 50 C water to run district heating, about half of the energy stored in near boiling water cannot be utilized. This is much less with 600 C sand.
And 50C is too low. That is minimum temperature of the heated tap water(Legionella and other diseases). And preferably you want some higher. And then as distance increases there are losses and other people using the heat. So temperature you need is actually quite high and in very cold days can be over 100C...