Who would want to merge into the $17 trillion in public debt, the economy strictly dependent on 0% interest rates (aka inflation), the poverty, the vast welfare expenses (that we can't afford), the collapsing infrastructure, the 14% unemployment, the $700 billion in military expenditures, the 3/4 of a trillion dollar budget deficit, the hundreds of billions in trade deficit, and the $100+ trillion in entitlement liabilities - that America is drowning under? Canada lacks most of those problems currently, and certainly the relative scale of those problems.
Why would Canada want to give up its countless unique national attributes? It would be smothered in any merger, and would see its national identity wiped out by a very obnoxiously aggressive US Federal Government.
Canada is a far more polite, gentle country at a government level. America is a violent, aggressive, quasi police state.
Canada's cities are some of the nicest on earth. The majority of large American cities are a wasteland of poverty and crime (10 out of the top 100 of the most violent large cities).
Canada has nothing to gain and everything to lose by merging up. As it is they get the benefits of being next to the largest economy on earth, with none of the negatives of having to be governed by the politicians in DC. It will never happen.