I have a tremendous interest in aviation and love to read,
so why don't the aviation magazines interest me anymore? Have the
magazines changed? Is it me? Is it that the subject matter is finite and
after reading 20 years worth of aviation magazines, there is very little
left that is new and interesting to me?

Yes, the subject matter has changed. It has shifted to high-priced
aircraft and avionics. Although, there aren't many aircraft today that
aren't high priced, relatively speaking.
Also, if you go back through the years, there is a five year cycle or
recyle of magazine topics. Every five years, the topcs are updated to
reflect the change in technology.