In part, it was the global economy.
Many small farmers in the German-speaking regions of Europe in the mid-1800s
made necessary supplements to their incomes by cottage industries. One was
weaving linen cloth. Slavery in the southern USA produced much cotton fiber
cheaply. The Industrial Revolution in Britain converted the fiber to good,
inexpensive cloth. As a result of the competiton, home-weaving of linen
fabric thus became unprofitable.
The potato famine reached continental Europe at about that time. It was not
so devastating as in Ireland, where it was about the only crop and where
there was political effort to starve out the Irish, but it had an impact.
None of the late-1840s pro-democracy revolutions on the Continent succeeded.
Emigration thus became an encouraging alternative for many.
The Missouri territory was at that time one of the attractive places to
which to emigrate. Land was good, and cheap, while land in the eastern
states was becoming crowded; indeed, people from the middle-Atlantic states
were migrating, first to Kentucky and Tennessee, then to Missouri.
Cotton carriers, headed back to USA for their next load, carried immigrants
at low fare.
Steamboats made it fairly easy to get from New Orleans to Missouri.
The main USA railroads were built only after the Civil War (1861-1865).
Duden had visited Missouri when the weather was outstanding; he urged (Eine
Reise nach Amerika) people to emigrate to Missouri. Sorry, we have not had
such good weather since he left.
Thus, mid-Missouri was settled by German speakers.
Bob Doerr
Please see http://www.rollanet.org/~bdoerr/state.htm