Pure economics.
Bricks were cheaper to make than stone was to quarry, dress and transport.
Simples!
If only it were that simple tho like...
What I noticed was that many (I'd say 80%) of the old mine chimneys have a bit stuck on the top... like they were finished and extended later...
I could understand one or two perhaps finding that they'd built it too small but not on such a wide scale...
I wondered if it could be a change of fuel (peat to coal?) that would require a large flue across the entire region...?