This is just speculation on my part but I think the 200 day simple and
the 40 week simple were some of the early indicators used.

The fact that no computers were available made them bypass "complex"
and go for "simple".

After years of these things being used successfully no one wanted
"to mess" with what worked, so I believe many traders just continued
using simple.
So many of the "old school" type analysts (that you would see on CNBC)
would probably use a 200 day simple.

Today you will typically hear a trader say the ema is better because its more
sensitive to recent data. However, I've never seen anyone really prove they
made or lost more money using either.

One thing to be aware of, there is a "window" issue with simple
averages, when values enter and drop out of the smoothing window.

Lets say you have a 3 period average and the values are:

89, 100, 105

this will average about 100

Then a large value enters the smoothing:

100, 105, 10000

the old 89 data drops off and 10000 enters the average,
now the value is about 3400 because of the 10000.
(it swamps the other numbers)

next we get this:

105, 10000, 102

the old 100 drops off and we get 102 the average is still the same
about 3400 because of the 10000.

next we get:

10000, 102, 110

again its about 3400 mainly because of the 10000

finally the 10000 drops out and we get:

102, 110, 80

now we are back to an average of about 100 because the 10000
is 4 bars away. (it fell out of the smoothing window)

So with simple smoothing there can be issues based on a large
value being in the smoothing.

This would typically occur in volume based indicators like the Force Index.
Where values can be quite large or small relative
to each other.

Thanks
diceman