Why is good heat transfer required for making fond?
I think it's a simple as bad heat transfer often meaning uneven heat transfer, leading to burning. Burnt fond won't deglaze successfully. Compare deglazing with making a roux: in both cases you want the proteins and starches to be browned (by the Malliard reaction and caramelization) but not burnt.
When making fond you want relatively high heat to promote these reactions and avoid the food (particularly some watery vegetables) from just sitting there in a lightly steaming puddle.