In short, Canada is a large landmass that is difficult to march massive armies through. Wellington gave his opinion that control of America largely relied on control of the St.Lawrence and Mississippi rivers (an opinion somewhat prove by the ACW). He also commented that Britain simply didn't have the resources to do this in the harsh, fairly unforgiving terrain of North America (compared to Europe).
I think for the U.S. to re-initiate hostilities after the Treaty of Ghent was signed would have been a bad idea. Not only would it mean more years of dealing with the Royal Navy interfering with trade, but if the Americans did attack after New Orleans, the British would have quite a bit of legitimacy as the wronged party if America broke the Treaty of Ghent. Add to this the loss of face to American statesmanship, and the fact that Britain had a battle-hardened army under a brilliant commander who probably could have defended Canada as effectively as Lippe defended Portugal.