Why does the fusion of heavier atoms require higher temperatures than
the fusion of hydrogen?

http://qso.as.arizona.edu/~ckeeton/NATS102/swork1.htm#heavy "How Stars
Work" by Chuck Keeton

"However, fusion between heavier elements requires even higher
temperatures.  ...[H]igh temperatures are needed to make the nuclei
fly around fast enough that they can overcome their electrical
repulsion. Heavier nuclei have more charge than hydrogen nuclei, so
they repel each other more strongly. ...Hence they have to be flying
even faster in order to overcome the repulsion. For example, hydrogen
can fuse above 7 million degrees; but helium can fuse only above 100
million degrees.

We saw above that fusion releases energy because some mass gets turned
into energy. However, this stops at iron. If you try to fuse iron or
any elements heavier than iron, you will find that fusion does not
release energy."

Sincerely,

Wonko

Excellent answer!  Thanks