Why was the land west of the Appalachians was an important factor in the coming of the American Revolution?

The way that this land was most connected to the coming of
the Revolution was through the Proclamation of 1763.  In this proclamation, the British
government told the colonists that they could not settle beyond those
mountains.

The British had their reasons.  They did not
want to anger the Indians and they did not want to have to pay to defend the colonists
from angry Indians.  However, these reasons did not resonate with the colonists. 
Instead, the colonists became angry.  They felt that the government did not care about
them and was, instead, going to tax them but then turn around and refuse to protect
them.  This helped to start making the colonists angry at the British and it helped to
(eventually) lead to the Revolution.