well...
when you looked at the original minuteman concept, it also included a semi-truck type launcher and a rail road container launcher option. Unfortunately, the US military also reconized that we have these large free reign open territorys that cant be guarded against every tom dick and harry that wanted a free minuteman warhead, large mobile support teams to guard the missile was prohibitively expensive, so the project was scrapped.
heres a link to the container concept. this later went straight to the transporter and errector that serviced the missiles.
http://www.nps.gov/mimi/history/srs/fig3-17.jpg
and the rail concept
http://www.strategic-air-command.com...le_History.htm
Now the soviet union was a little different: not so much uncontrolled travel, the use of large guard teams was practical to support the missiles, and large tracts of land to always make the missile a little harder to find makes the mobile launcher in their case a better option.