Thermobaric and other large bombs, why not use them?

[Greg]

Rather than expend men and machines on the heavily defended Avdeevka, why would Russia simply not carpet bomb it with overwhelming explosive power?  It is a small town.

You have a small city 30,000 people (that is pretty small) with trenches, pill boxes etc and many many of them as well as layered minefields but none of those things are air tight.  Would several dozen thermobaric bombs dropped simultaneously not kill so many of the inhabitants of those pill boxes and trenches that you could then overrun the injured survivors?

There are zero civilians in the town

Another option would be an underground nuclear bomb like an underground nuclear test.  Would that not liquify the ground in winter when it is saturated, collapse the tunnels and kill most of the people in the small town?  I thought a bomb in the ground would do that.

I am not super familiar with large ordnance.  More of an aerosol can explosives expert.