I would have to guess your memory has exaggerated the size of the insect. A 10" praying mantis is not impossible, but it's certainly unheard of and
counter-intuitive.
Gigantism can occur in just about any species, but insects are poorly suited to growing to extreme proportions because of the mechanics of their
bodies. Things like air intake, fluid circulation, membrane integrity, the efficiency of these systems does down rapidly as size increases. Reason
being, the external dimensions grow at a much slower rate (1/2 I think), proportionally, than the internal mass. If the insect's surface area grows
by x, then the internal cavity grows by 2x, right?
Also, the mantis is not the hardiest of insects, and their slow disappearance from America is definitely reality. You can still find them in
abundance overseas, but the environment and the biodiversity in North America really isn't sufficient to support a big mantis population. Also,
they're very sensitive to temperature changes, and the fact that even the southernmost states experience frost and freezing temperatures pretty much
clinches it.
They're not the easiest insect to take care of, even in captivity, and I think they just weren't up to the challenge of adapting to
industrialization, wilder temperature shifts, predation by rats and birds, and of course an unstable food supply. They eat quite a bit for their size
if I remember, and have fairly large territories. I could be totally wrong, over the years the old coconut has lost much of its milk...