Art KB9MZ wrote:
"Why must only series circuits be considered for radiators?"

It is convenient to follow custom.

Art also wrote:
"What is it about parallel circuits that make them unsuitable?"

We are more familiar with impedance than admittance in most cases due to
our instruction.

A parallel resonsant circuit is a high impedance (low admittance).
A series resonant circuit is a low impedance (high admittance).

For a parallel circuit, Q=R/X.
For a series circuit, Q=X/R.

The series circuit has its equivalent parallel circuit.
The parallel circuit has its equivalent series circuit.
The mathematical conversion from one to the other is simple and shown in
my 19th edition of the ARRL Antenna Book on page 24-12.
(By coincidence the action of 1//4-wave and 1/2-wave transmission line
sections is shown on the next page, 24-13) Commentors on using a
1/2-wave of foreign line inserted in a 50-ohm system, and what the ITT
Handbook says should consult the Antenna Book or some other book that
not only gives you the basic information but also tells you how to use
the information.

Best regards, Richard Harrison, KB5WZI