Why did the use of DNA polymerase from the bacterium THermus aquaticus
allow researchers to add the necessary reagents to tubes in a
preprogrammed heating block (PCR)?

Hey, Great Question!

The Taq polymerase is heat resistant, which is key, as it can be
raised to a temperature beyond its functional range to allow for the
melting of the primer and copy strands in the chain reaction.
This is repeated through many cycles, each time exponentially
increasing the number of copies of the target sequence.

Taq Pol made this possible because it isn't denatured at temperatures
which melt dsDNA.