Why is it that medium to well paying professions (e.g. Nurses,
Policemen, Power workers , Airline Pilots) tend to have powerful
unions to support/represent them, while low paying professions (retail
workers , Call Centre operators) have NO unions to protect them
allowing them to be kicked around at the employers' mercy?

What is the fundamental reason why these latter groups fail to unionize?

Mongolia

The main reason that I can see is that the first group of workers are
professionals, i.e. they had to have specialized training for their
jobs. The second group's jobs can be done by anyone with people skills
and do not require any special training. In other words, they could be
easily replaced whereas a nurse or pilot cannot. If call centre
workers tried to unionize and receive better benefits and more money,
they would simply be fired and replaced with other people right out of
high school, eager to take over their jobs.  This is why workers at
Walmart would probably not be successful in attempts to unionise--
they would simply all be fired and replaced.

Another reason is Union Dues.. minimum wage barely covers (if) the
many bills and sometimes union dues are over 30-70 bucks.. but
ofcourse strong reason of easy replacement - unskilled position is
dominant

I'm not sure I can accept the premise of the question.  The SEIU
(janitorial) and UNITE (garment workers) are pretty influential in
their own right, and if anything, are probably on the upswing, while
the more middle class unions are waning fast.

It's probably more the case that certain jobs offer decent wages
because they are unionized, rather than it being that only well-paying
jobs are the ones that get unions.

paf

In addition, I believe it also relates to when the unions became
organized and the type of employers.  Decades ago, uniona were
stronger and able to organize members in firms.  Once established as
negotiating party with the employers, the interest of new employees to
join was/is strong.

These days in America, unions are weaker, less respected, and the
ability to organize has declined.  Furthermore, trying to organize in
the very fragmented industries with minimum wage employees is more
difficult than in the days when low wage employees could stop
production in a steel mill, railroad traffic, etc.